It's very cold today.But it's ----than it was yesterday A less cold B less colder Cnot less colder

选A.
首先就应该排除B,C.less 后应接形容词的原级.
附less的用法：
less
a.(little的比较级)
1.较小的,较少的[+than]
He has less strength than I.
他力气比我小.
ad.(little的比较级)
1.较小地,较少地;不如[+than]
Thunderstorms are much less common in Ireland than in England.
爱尔兰的雷雨远较英格兰为少.
n.
1.更少的数(量,额)[U]
I could do no less.
这是我起码要做的.
prep.
1.减去;扣除;差...
She stayed a month less two days.
她呆了一个月少两天.