Which is screw onto a  bolt to fasten things yogether

Although Margaret Mead had several assistants during her long investigations of  Samoa, the bulk of the research was done by ……… alone. 

Over 1900 years ago, ………, who worked in Alexandria, Egypt, classified the stars  by their brightness.

Gradually, Microsoft became the ……… company in the software business. 

معنی ضرب المثل  " a donkey that carries a lot of books is not necessarily learned"