The road conditions there turned out to be very good, _____ was more than we could expect. ...

The road conditions there turned out to be very good, _____ was more than we could expect.
[     ]

A. it
B. what
C. which
D. that

C

which在这里引导非限制性定语从句，充当从句的主语，what是不可以用在定语从句中的，无论是非限制性还是限制性。what可以再宾语从句，表语从句等其他的从句中充当句子成分。