Temperature or temperatures?

English as a Second Language English Vocabulary, Grammar and Idioms
#1

What variant is the correct?
“The average temperatures are −4 in January
and +20 in June.”
“The average temperature is −4 in January
and +20 in June.”
Tnx.

#2

Both are fine.

#3

Correct. The noun and verb form must correspond, i.e., plural with plural, singular with singular.