―________ fine day it is today!

There is a deadlock(僵局) in the discussion when neither side gives ________ to the other.

Lying under the skull, the human brain contains an average of ten billion nerve cells called ______.

Thisisanimportantproblem_______toofewsocialscientistshaveresearched.

This is an important problem _______ too few social scientists have researched.