Both the police and the workmen were grateful to the student for _______ in advance.   [ ]

A. they had been informed this    

B. their informing of this

C. informing them of this        

D. having informed them this

研究表明，小学生道德感发展的转折期是（）。

A.三年级

B.四年级

C.五年级

D.六年级