生物的遗传和______是生物进化的基础。
变异
一个长方形周长为30cm,若长与宽的比为2:1,则长方形的宽为______厘米.
A cat, ______ it is a white one ______ a black one, is considered to be a good cat as long as it can catch mice.
A.both; and
B.not only; but also
C.either; or
D.whether; or