They to put their hearts into study..
A. have made their minds
B. have made their mind
C. have made up their minds
D. have made up their mind
网友回答
A. have made their minds
B. have made their mind
(答案→)C. have made up their minds
D. have made up their mind
解析：考查短语固定搭配。本题考查短语固定搭配make up one’s mind下定决心做某事。mind的数随物主代词的数而变化。句意：他们已经下定决心投身于学习中。故C正确。