Explanation
Correct answer: A.
Choice A is the best answer because it most effectively uses data from the graph to support the researchers’ recommendation that Sururu shell aggregate (SSA) substitution be done in relatively small proportions to avoid compromising the compressive strength of mortar mixtures. The text explains that a research team investigated whether SSA can serve as a substitute for sand in mortar mixtures. The graph presents the compressive strength (in megapascals) of mortar mixtures with SSA replacing 0% (control), 10%, 20%, and 40% of the sand on day 7 and day 28 of curing. The graph shows that the compressive strength of the 10% SSA mixture was approximately 25 megapascals on day 7, which is greater than the control mixture’s approximately 22 megapascals on the same day. The graph also shows that the 10% SSA mixture’s compressive strength was slightly more than 25 megapascals on day 28, which is only slightly less than the control’s compressive strength on the same day (about 26 megapascals). Therefore, the graph shows that the compressive strength of the 10% SSA mixture was greater than that of the control on day 7 and roughly equal to that of the control on day 28, suggesting that substituting only 10% of the sand with SSA avoids compromising the mortar’s compressive strength. The graph also shows that on day 7, the 20% and 40% SSA mixtures had compressive strengths of about 19 and 13 megapascals, respectively (both lower than the control’s), and on day 28, they had compressive strengths of about 22 and 15 megapascals, respectively (once again lower than the control’s). Thus, the data support the recommendation by indicating that substituting relatively small proportions of sand with SSA in mortar mixtures doesn’t compromise strength compared with the control, whereas substituting larger proportions does.
Why the other choices are wrong
Choice B
Choice B is incorrect because the comparison it makes isn’t relevant to the researchers’ recommendation. Although the graph does show that the bars representing the compressive strength of the 10% SSA mixture on day 7 and day 28 are roughly the same height (suggesting that the mixture’s compressive strength changed by only a small amount), and although there are relatively large gaps between the bars showing the compressive strength of the 20% and 40% SSA mixtures on day 7 and day 28 (suggesting that their compressive strength changed over time by a greater amount), this comparison concerns only how each SSA mixture’s strength changed over time and doesn’t compare the SSA mixtures with the control mixture. Supporting the recommendation that SSA substitution be done in small proportions requires evidence that small substitutions don’t compromise strength relative to the control while larger substitutions do.
Choice C
Choice C is incorrect because it doesn’t accurately represent the data in the graph, nor is the comparison it makes relevant to the researchers’ recommendation. While the graph does show that the compressive strength of the 10% SSA mixture was greater than that of the control on day 7 (about 25 megapascals vs. about 22 megapascals) and, moreover, that the compressive strength of the control increased from day 7 to day 28 (from about 22 megapascals to about 26 megapascals), the graph shows that the compressive strength of the 10% SSA mixture increased by a small amount (from about 25 megapascals to slightly more than 25 megapascals), not that it decreased. Moreover, the researchers recommended substituting relatively small proportions (compared with larger proportions) of the mixtures with SSA; comparing the compressive strength of the 10% SSA mixture with that of the control without also considering how the compressive strength of the 10% SSA mixture compared with that of the 20% and 40% SSA mixtures wouldn’t support the recommendation.
Choice D
Choice D is incorrect. Though the graph indicates that the 10% SSA mixture’s compressive strength on day 7 and day 28 (about 25 megapascals and slightly more than 25 megapascals, respectively) didn’t surpass the control mixture’s compressive strength on day 28 (about 26 megapascals), this observation doesn’t support the recommendation. The recommendation concerns the trade-off between SSA proportion and compressive strength; supporting it requires not just evidence about how the 10% SSA mixture compared with the control but also evidence about how the mixtures with larger proportions of SSA (20% and 40%) compared with the 10% SSA mixture and the control.