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