1.

a) I  cut my finger and Juan injured his hand

b) Juan injured his hand but it wasn't serious

c) We called the medic because Juan injured his hand

2.

a) We cleaned the spark plug and replaced the gasket

b) It's running much better but it's making a funny noise

c) It's running much better because we cleaned the spark plug

3.

a) It isn't damaged and we are using this tank

b) It's not damaged but we aren't using this tank

c) We are using this tank because it's not damaged