The explanation, in every case, is homework that was never turned in. Missing homeworks count as zero, and zero really pulls down the average. This is in accordance with course policy. If there are homework assignments that you did not turn in, you are being given a second chance. You must complete them and turn them in no later than the lecture on Wednesday, May 2.
Homework assignments are available on the course web-page.
There is no extra credit option in this course. If you are unhappy with your performance, however, you have the following option. You can write a paper, and the grade on the paper will replace your worst exam grade. This is not extra credit, because you don't automatically get extra points for writing a paper. You need to write a good paper, because that will improve your grade.