What's the difference of then and than?

Geoffrey is writing a paper investigating friendly-fire incidents in recent us military actions. which paragraph topic would be relevant to his paper and would therefore merit inclusion in his final draft? a) a paragraph criticizing us military actions in the viet nam war b) a paragraph discussing how friendly-fire incidents are investigated c) a paragraph describing what basic boot camp is like for a new army enlistee eliminate d) a paragraph discussing how hard an extended military service can be on a family