Research Shows That, Across Time, Married Couples Have Sex Less

Research shows that, across time, married couples have sex less and less often. However, it should be noted that the ____. ​

A) lower frequency can be almost entirely explained by the fact that longer-married couples tend to have children (and having children has the effect of reducing sex) .
B) decline is statistically insignificant and cyclical in nature
C) lower frequency is due to both age and length of marriage
D) decline does not start for several years into marriage