English is the language used by speakers of many countries.

English is the language used by speakers of many countries.
1.It's a truck only _____ on farms.
A.used.B.using.C.to use.
2.English is the language （used） by speakers of many countries.
3.For many countries,Latin was the common language between （educated）people in Europe.
请问这三道题,第一题为什么用used 什么句型,做什么成分.第二题,第三题也是.