为什么不选when As a new graduate,he doesn’t know ----- it takes t

为什么不选when As a new graduate,he doesn’t know _____ it takes to start a business here.
A how Bwhat C when D which 我知道what 在这里做take 的宾语,但是从字面意思来讲when 也说的通啊?请问when 有这个用法吗?