单选题It is difficult to send people to other planets because of ______.A lack of capacity of space exploration vehicles.B the financial expenditure involved in space travel.C controversial nature of space travel.D the uncertain future of space exploration.
题目
单选题
It is difficult to send people to other planets because of ______.
A
lack of capacity of space exploration vehicles.
B
the financial expenditure involved in space travel.
C
controversial nature of space travel.
D
the uncertain future of space exploration.