Exactly when the first people arrived in we now know as California no one really knows.A

问题补充：

Exactly when the first people arrived in we now know as California, no one really knows.

A. which

B. what

C. that

D. it

答案：

A. which

(答案→)B. what

C. that

D. it

解析：考查宾语从句。宾语从句What we now know as California作为介词in的宾语。What引导起该宾语从句，在句中作为动词know的宾语。