We can always see how much time has passed when appreciating old photos,_____

Never give up! ____ is impossible if you put your heart in it.

Many scientists have believed that theremay be a _________of life on the Mar．

.It was in the 1990s _______ we were working together ____ we made friends with each other.

In order to reduce the increasing prices of vegetables，the toll if a truck carrying them passes through a toll sration.