As nobody here knows what is wrong with the machine, we must send for an engineer to ____ the problem.

Sam _____ some knowledge of the computer just by watching others working on it.

Some insects_______ the colour of their surrounding to protect themselves.

In order to ____ the debt, the couple has worked hard for years with little food and only a small room to live in and never a moment's rest.

When everybody was listening carefully, the noise of several people fighting outside____ the professor’s speech.