UP TO NOW, _______ SPACE EXPLORING STILL REMAINS _______ VERY RISK...

17. - 25 DE THI HOC KI 1 MON TIENG ANH LOP 12 CO DAP AN
25 DE THI HOC KI 1 MON TIENG ANH LOP 12 CO DAP AN

17.

Up to now, _______ space exploring still remains _______ very risky, complex, and expensive

endeavor. [endeavor: effort]

A. the / the

B. a / the

C. a / Ø

D. Ø / a