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子网掩码是由位二进制数组成，它的子网主机标识部分为全“”

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Directions: There are 15 sentences in this section. For each sentence there are four choices marked A, B, C and D. Choose one answer that best completes the sentence and blacken the corresponding letter on the answer sheet.
6. ______ you go there early, there won't be much a chance of getting a ticket.

A. Until
B. If
C. Unless
D. If only

[A] to be doing
[B] with doing
[C] to do
[D] of doing

We shall probably never be able to _____ the exact nature of these sub-atomic particles.
[A] assert
[B] impart
[C] ascertain
[D] notify

[A] respond
[B] return
[C] retort
[D] reply