Explanation
Correct answer: C.
Choice C is the best answer because it best describes the function of the underlined portion in the text as a whole. The text establishes that scientists developed a pellicle (a dense cellulose mesh) as an ecologically sustainable alternative to petroleum-based synthetic leather, noting that the pellicle is leatherlike except in color. The text then notes that the standard dyeing process for leather generates wastewater and other byproducts, and the underlined portion states that using such a process would be inconsistent with the ecological promise of the pellicle approach. The sentence that follows the underlined portion describes how Walker and colleagues addressed this problem by modifying bacteria to produce a pellicle with embedded pigmentation. Thus, the underlined portion describes a consideration—the incompatibility of standard dyeing methods with ecological aims—that led the researchers to employ an alternative coloring approach consistent with their goal of ecological sustainability.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text presents the underlined portion not as a concession that the researchers’ main goal, developing an ecologically sustainable alternative to synthetic leather, will be difficult to achieve, but as a reason why adopting a particular method of dyeing would be inconsistent with that goal.
Choice B
Choice B is incorrect because the underlined portion addresses a problem with the standard process for dyeing leather, not a characteristic of conventional synthetic leathers themselves. The issue identified in the underlined portion concerns the environmental impact of the standard method used to dye leather, not the petroleum-based nature of conventional synthetics.
Choice D
Choice D is incorrect because the underlined portion identifies the problem that motivated the researchers’ search for an alternative method of coloring the pellicle, not the method of coloring itself. The method of modifying bacteria to embed pigmentation is described in the sentence that follows the underlined portion.