30．teamwork． A．where B．which C．that D．what

30．teamwork． A．where B．which C．that D．what
30．It was today's activity let me know the importance ofteamwork．
A．where B．which C．that D．what
这题为什么要填that? 是什么同位语从句么? 如何正确来解释这个题目呢?