All animals in nature must learn to        to a changing environment in order to survive.

The number of people invited ______ fifty, but a number of them ______ absent for different reasons.

It's OK to________an electrical appliance on so long as you are using it.

Try not to cough more than you can _______ since it may cause problems to your lungs. 

Eye doctors recommend that a child’s first eye exam ________ at the age of six months old.