i can't understand________ at the poor little girlA they laughB their laughingC them to laughD them why laugh

i can't understand________ at the poor little girlA they laughB their laughingC them to laughD them why laugh
i can't understand________ at the poor little girl
A they laugh
B their laughing
C them to laugh
D them why laugh

i can't understand________ at the poor little girlA they laughB their laughingC them to laughD them why laugh
understand sb.to do sth.理解某人做某事.

B

b

b 把understand 后面的都看做一个整体 所以their 后面用动名次形式

B understand动词后接动名词

B their laughing