---- ______plastic bags we use, _____ it will be to the environment.

---- I agree ________ you.

A．The less; the better; with

B．The fewer; the better; with

C．The less; the worse; to

D．The fewer; the worse; to

假设随机变量X与Y的相关系数为ρ，则ρ=1的充要条件是

A．Y=aX+b(a＞0)．
B．cov(X，Y)=1，DX=DY=1．
C．cov(X，Y)=

，

=

． D．D(X+Y)=

．