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relative clause

relative clause

I confuse using of "which"and "when "in this statement:
"My computer has a coffee......Iuse at lauchtime"
And"A post office is a place........We can buy stamp"
The choices:

a,which
b,where
c,when
d,why
I chose the second (where)for both.In fact the right are:
St1:which
St2:where
i can not understand.Could you tell me the differences?
Thank so much!