I suggest collecting all the (a)   letters, and then send them directly to the head of the committee. (complain)

In order to keep our environment (a)   , we must solve lots of problems. (pollute)

Some volunteer (a)   help to clear up the beach. (conserve)

It is (a)   that our environment is more and more polluted. (disappoint)

She can save much water by taking showers instead of baths. (Unless...)

(a)  

The river is too dirty for us to swim. (Since...)

(a)  

Hard work leads us to success. (If...)

(a)  

Mr. Brown was disappointed because his son didn’t write to him. (Mr. Brown's son ...)

(a)  

I can’t drive because I haven’t passed the driving test. (I can't drive because of...)

(a)