Relative Clause ( what, where)...

We're currently in _____ we might call the optimum period, ______ the world is warmer than it has been for the last 20,000 years.

A) which/when
B) that/how
C) which/that
D) what/where
E) that/when

Hi, here, I went for "D".

Can we use "where" for the optimum period here? If not, why?

Thanks a lot in advance.

All right, if we changed the options A and D just like this, Which one would it be true? And why?

We're currently in _____ we might call the optimum period, ______ the world is warmer than it has been for the last 20,000 years.

A) what/when
B) that/how
C) which/that
D) what/where
E) that/when

Thanks alot in advance

Among the answers, A is the preferred answer, although "where" may be accepted. Anytime when you have a main sentence with time base element preceeding an indenpent clasue (i.e. hour, day, and any period), "when" is the appropriate conjunction.