|Question 2Verbal

Source Texts

Text
Because cryptophytes and other microphytobenthos (MPB)—microscopic organisms inhabiting tidal flats—are consumed by clams as well as by certain shorebirds and fish, MPB are ______ estuary ecosystems. Decreased MPB abundance, which may be caused by environmental stressors, thus has a direct, negative impact on coastal food webs.
Which choice completes the text with the most logical and precise word or phrase?
imperative for
A
interchangeable with
B
emblematic of
C
subordinate to
D