a,平均
b,平常的,强调有规律的
c,普通的,一般的
d,正常的
所以选d
normal
a. 正常的,正规的
n. 正常状态
侧重是否正常
I really don't know what kind of man he really is; on one occasion, he is normal, and on another, he acts like a lunatic.
我真不了解他到底是怎样的人；一会儿正常,一会儿又像个疯子.
usual
a. 通常的,often
The vintage was later than usual.
那次葡萄采收期比平常年份晚.