B strengthens the argument by discounting meteorites being a possible cause for the straight line of craters. This makes the conclusion all the more likely that it was the result of volcanic events.
A doesn't strengthen the argument because it isn't close enough to the scenario presented in the passage. It should be a long straight line of craters. If this was proven to be due to volcanic events, then this would strengthen the argument. However, showing that volcanic events were responsible for a short line of craters doesn't help. It also neglects the fact that the craters were all different ages in the passage.
I hope this helps. Please let me know if you have any other questions!