Research suggests some students _ learn better in singlesex

Research suggests some students _ learn better in singlesex education environments.
Research suggests some students () learn better in single-sex education environments.
A may
B would
C should
D must
为什么选a不选b?
——up early in the morning to have a walk ,I am sure,you are likely to feel active all day
A Get BHaving got CGetting DTo get
为什么选C不选A?