Which is not a technique for better understanding a subject

The correct word to complete the sentence

Insert comma today we went to the store to the post office and to the park

Which choice is a major detail from paragraph two? a) the great wall is a large network of several walls and towers. b) the great wall looks like a long wall snaking its way across china. c) the great wall was constructed over hundreds of years beginning in 221 b. c. d) the great wall construction encouraged several independent kingdoms to unite.