Black holes can not be seen directly, so
题型:选择题
问题:
Black holes can not be seen directly, so determining the number of them is a___________ task. [ ]
A. rough
B. strict
C. tough
D. firm
Black holes can not be seen directly, so determining the number of them is a___________ task. [ ]
A. rough
B. strict
C. tough
D. firm