一个物体受到两个共点力的作用，无论是在什么情况下，其合力( )。
A. ( 一定大于任意一个分力
B. ( 至少比一个分力大
C. ( 不大于两个分力大小的和，不小于两个分力大小的差
D. ( 随着两个分力夹角的增大而增大
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Because of the high blood pressure, the doctor advised him to ______ from smoking.
A. constrain
B. restrain
C. refrain
D. remain
He felt like a small boat on a(n) ___________ ocean.
A. enormous
B. huge
C. gigantic
D. vast
Who would be pardoned according to the news
A. Those having no connection with recent bombings.
B. Those who give up their weapons voluntarily.
C. Those who cooperate with the government.
D. Those who killed the American hostage.
u=f（x，y），x=rcosθ，y=sinθ，f具有一阶、二阶连续的偏导数，则urθ（）。
A. -fx·sinθ+fy·cosθ
B. rsinθcosθ(fyy-fxx)-fx·sinθ+fy·cosθ
C. rcos2θ·fxy-fx·sinθ+fy·cosθ
D. rsinθcosθ(fyy-fxx)+rcos2θ·fxy-fx·sinθ+fy·cosθ
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