The other passengers will get on the bus soon. Then we'll leave.

A. The possibility of changing the Martian environment

B. The challenge of interplanetary travel

C. The advantages of establishing colonies on Mars

D. The need to study Martian ecology

B. lacks

C. hopes

D. neglects

A. The work I do is harder, so I get better grades.

B. I work harder than before even if I get better grades.

C. The harder I work, the better grades I get.

D. The work I do is better, so I work harder.

A. appealing

B. committed

C. specialized

D. admitted

A. has read / make

B. was reading/made

C. had read/made

D. read / had made

A. humanity

B. civilization

C. constitution

D. philosophy

A. hadn't received / moved

B. don't receive / had moved

C. didn't receive / have moved

D. haven't received/moved