> 
> Jane is back in May, by\_\_\_\_ the new house should be finished.
> 
> 
> A. which B. that C. whom D. when
> 
> 
> 

"when" is the answer, and I know its grammar.

My question is whether or not "which" works and why.

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relative question: [Stephen Hawking believes that the earth is unlikely to be the only planet \_\_\_\_ life has developed gradually](https://ell.stackexchange.com/questions/165175/stephen-hawking-believes-that-the-earth-is-unlikely-to-be-the-only-planet-l)