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Black holes not be seen directly, so determining the number of them is a tough t

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Black holes not be seen directly, so determining the number of them is a tough t

(2006·上海)

Black holes ________ not be seen directly, so determining the number of them is a tough task.

[]

A．canB．shouldC．mustD．need

试题答案

答案：A

标签：sodirectlythedeterminingseen

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