a, Because the water is highly polluted, we can not swim in this part of the river.

b, The company does not produce recycled paper as because it is more expensive to produce than regular paper.

c, Since we want people to be aware of the problem of pollution, we have "a green week" three times a year.

d, Because the area around the beach is full of trash, no one wants to go swimiing or sunbathing there.

e, As it started to rain, we couldn't keep on cleaning the beach.

f, Since Mr Minh is admired, he dedicated all his life to protecting enviroment.

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a, I was surprised that no one stopped those people cutting down the trees.

b, People are worried that the oil will pollute the sea.

c, I am proud that my team has won the first award of the competition.

d, They are certain that they can help us clean up the beach.

e, I am very happy that you could recover very quickly.

f, I am disappointed that they didn't tell us how to do it.

2. Join the pairs of sentence together. Use "because, as or since'
a. We can not swim in this part of the river because the water is highly polluted. 
b. The company does not produce recycled paper because it is more expensive to produce than regular paper
c. We have "a green week" three times a year because we want people to be aware of the problem of pollution. 
d. Noone wants to go swimming or sunbathing there because the area around the beach is full of trash.
e. We could not keep on cleaning the beach as it started to rain  
f. Mr. Minh is admired because he dedicated all his life to protecting environment.
3. Complete the sentences, using "that".
a. I was surprised that no one stopped those people cutting down the trees. 
b. People are worried that the oil will pollute the sea. 
c. I am proud that my team has won the first award of the competition. 
d. They are certain that they can help us clean up the beach. 
e. I am very happy that you could recover very quickly. 
f I am disappointed that they did not tell us how to do it.