P,Q为可逆矩阵, 则 r(PA)=r(A)=r(AQ)=r(PAQ)
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You had better book rooms at the hotel in advance ______ you should find no room on your arrival.
A. so that
B. even if
C. in case
D. now that
_______ computer games bring us a lot of pleasure, I don't think it wise to spend too much money and time on them.
A. Because
B. As
C. While
D. If
_____ you are unable to deal with it, perhaps we should ask someone else for help.
Although
B. While
C. Since
D. Unless
_______ I have to give a speech, I get extremely nervous before I start.
A. Whatever
B. Whenever
C. Whoever
D. However