[单项选择]某随机事件最多只有x、y、z三种互不相同的结果，关于x、y、z发生的概率，以下有可能的是______。
A. P(X)=1，P(Y)=-1，P(Z)=1
B. P(X)=0.3，P(Y)=0.2，P(Z)=0.5
C. P(X)=P(Y)=P(Z)=1
D. P(X)＞0，P(Y)=-P(X)，P(Z)=1

In the past few decades, remarkable findings have been made in ethology, the study of animal social behavior. Earlier scientists had (1) that nonhuman social life was almost totally instinctive or fixed by genetics. Much more careful observation has shown that (2) variation occurs among the social ties of most species, showing that learning is a part of social life. That is, the (3) are not solely fixed by the genes. (4) , the learning that occurs is often at an early age in a process that is called imprinting. Imprinting is clearly (5) instinctive, but it is not quite like the learning of humans; it is something in between the two. An illustration best (6) the nature of imprinting. Once, biologists thought that ducklings followed the mother duck because of instincts. Now we know that, shortly (7) they hatch, ducklings fix (8) any object about the size of a duck and will henceforth follow it. So ducklings may f
A. on
B. with
C. in
D. within