I Picked C can someone please help me !!!!!

Thanks

Choose the sentence that is written correctly.
A. If you do not understand why you received a particular score on your assignment you should ask your instructor.
B. If you do not understand why you received a particular score on your assignment. You should ask your instructor.
C. If you do not understand why you received a particular score on your assignment, you should ask your instructor.
D. If you do not understand why you received a particular score on your assignment; you should ask your instructor.