下列说法正确的是[ ]

A.一个数的绝对值一定是正数

B.有理数可以分为正有理数和负有理数

C.互为相反数的两个数的立方仍互为相反数

D.正数的倒数是正数，负数的倒数是负数，任何数都有倒数

It never _____  humans until the mid­1990s that some animals can be cloned.

A．occur

B．is occurred

C．occurred to

D．occurred