The experiment, conducted by exobiology students at the University of Washington and enabled by the replication of Martian soil, was a follow-on to earlier NASA challenges, for asking researchers to show that earthworms could live in highly alkaline environments.


a. was a follow-on to earlier NASA challenges, for asking

b. were follow-ons to an earlier NASA challenge, which asked

c. were follow-ons to an earlier NASA challenge, for

d. was a follow-on to an earlier NASA challenge, for

e. was a follow-on to an earlier NASA challenge, which asked


OA E

Source: Veritas Prep