Question

a firefly uses specialized muscles to draw oxygen into its lower abdomen through narrow tubes, triggering a chemical reaction whereby the oxygen combines with chemicals in the firefly’s abdomen to produce a glow. ______ when the firefly stops drawing in oxygen, the reaction—and the glow—cease.
21 mark for review
which choice completes the text with the most logical transition?
a for instance,
b by contrast,
c specifically,
d in conclusion,

Explanation:

The sentence before the blank describes how a firefly produces light (when oxygen combines with chemicals), and the part after the blank describes what happens when the firefly stops drawing in oxygen (the glow ceases). This shows a contrast between the two situations. "By contrast" (B) is used to show a contrast. "For instance" (A) is for examples, "Specifically" (C) is for more details, and "In conclusion" (D) is for summarizing, which don't fit here.

Answer:

B. By contrast.