It is the test system, ________ the teachers, that is to blame for the heavy burden of middle school students nowadays.[ ]

Children who are not activeor__________diet is high in fat will gain weight quickly.

The air ______ different gases _____ together.

I don’t think _______ Maths is easy.

(2013·江西，26)There are a small number of people involved，possibly ________ twenty.