单项选择题

Which of the following description about ROC of z-transform is right？
A、
The ROC of z-transform means the range of n which makes the series of z-transform converges.
B、If the z-transform of a sequence exists, its DTFT exists.
C、If the ROC of the z-transform of a sequence includes the unit circle, its DTFT exists.
D、If a sequence x[n] is casual，its ROC must be out of a circle which includes the unit circle.

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通常衣服上会用哪个字母表示“中号”的衣服？

答案：正确答案：M

单项选择题

There is an old English saying. It tells us that we must go to bed early and get up early in the morning. Then we shall be healthy. We shall also be rich and clever. This is true. The body must have enough sleep to be healthy. Children of young age should have ten hours’ sleep every night. Children who do not have enough sleep cannot do their work well and they may not be healthy.
The body also needs exercise. Walking, running, jumping and playing games are all exercise. Exercise keeps the body strong. Exercise also helps the blood to move around inside the body. It is very important. Our blood moves to all parts of our body. The head also needs blood. Exercise helps us to think better.

A.
eat more B.
sleep more C.
play more D.
go to bed early and get up early