已知随机变量X的分布中含有若干个未知参数，如果仅对唯一的参数值才有EX=DX，则X必服从
A. 参数为(μ，σ2)的正态分布．
B. 参数为λ的指数分布．
C. 参数为λ的泊松分布．
D. 参数为a，b的[a，b]区间上的均匀分布．
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已知(X，Y)服从二维正态分布，且 EX=μ1，EY=μ2，DX=DY=σ2， ξ=aX+bY，η=aX-bY(ab≠0)， 则ξ与η独立的充要条件是
A. a、b为任意实数．
B. a=b-1．
C. a2=62．
D. a=b+1．
Which of the following statements is not characteristic of liquid-fuel rockets
A. The fuel is cheap.
B. They are cheap to build.
C. They can be stopped and re-ignited.
D. They must be used soon after fuelin
下列选项中不属于针叶树用途范围的是( )。
A. 制作斧柄
B. 制作屋架
C. 细木装饰
D. 制作檩条
Who are most likely to lie for serving purposes
A. physicians
B. surgeons
C. psychiatrists
D. lawyers
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