Sentence Correction

According to a recent study of sexual behavior in the U.S., over half of the couples that were married and engaged at the time of the study's survey were living together before making a final commitment to each other.

A) couples that were married and engaged at the time of the study's survey were living
B) couples that were engaged or married at the time of the study's survey had lived
C) couples who were engaged or married at the time when the study's survey occured were living
D) married and engaged couples at the time of the study's survey had lived
E) married or engaged couples at the time of the study's survey were living

Can you please explain why optionB is right and option D is wrong