The clever boy made a hole in the wall, _____ he could see _____ was going on inside the house. [ ]

A. which; what

B. through which; what

C. through that; what

D. what; that

我国是世界上重要的海洋大国，大陆海岸线长达多_________千米；我国陆上国界漫长，达__________万多千米，陆地相邻的国家_______个。