SAT Question 5aae2475: Boundaries answer and explanation

English - Boundaries - No Punctuation Within Clause
0:00
FREE OFFICIAL BANK
Stomata, tiny pore structures in a leaf that absorb gases needed for plant growth, open when guard cells surrounding each pore swell with water. In a pivotal 2007 article, plant cell ______ showed that lipid molecules called phosphatidylinositol phosphates are responsible for signaling guard cells to open stomata.
56
Difficulty:Hard
Which choice completes the text so that it conforms to the conventions of Standard English?