设二维随机变量(X,Y)服从二维正态分布,则随机变量ξ=X+Y与η=X-Y不相关的充要条件为
A. EX=EY
B. E(X2)- (EX)2= E(Y2)- (EY)2
C. E(X2)= E(Y2)
D. E(X2)+(EX)2=E(Y2)+(EY)2
Their appearance usually marks a cultural and economic renaissance, heralding a new golden age.
A. which indicates
B. which indicating
C. what indicates
D. what indicating
The focus on the city is significant, as are the ideas behind the recreation.
A. because the ideas behind the recreation are significant.
B. and the ideas behind the recreation are also significant.
C. while the ideas behind the recreation are significant.
D. though the ideas behind the recreation are significant.
Its entrepreneurs and scholars invented technologies still in use today...
A. when are still in use
B. where are still in use
C. what are still in use
D. which are still in use
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