1. The fire ___ due to faulty insulation. But, for the time being, we cannot exclude a terrorist attack.
A. may have started
B .might be started
C.could start
D. must have started
2. They _____ to the concert together because Jane had agreed to baby-sit for them.
A. could go
B.could have gone
C. were able to go
D. can go
I don't know the difference ''could '' and ''were able to go ''. Can you help me??