Understanding is one of the most important （） of a happy marriage.

A. participation in interesting activities relating to a school subject improves one’s achievement in that area
B. school principals do more than their share in interpreting the curriculum to the parents
C. only a small part of the school day should be set apart for drilling in arithmetic
D. too many children are lazy and have poor work habits

A. a mixed blessing
B. an evil force
C. an active stimulus
D. an intervention in their domestic affairs