Andrew, please   ___my e-mail as soon as possible.[ ]

A. write back                  

B. write to

C. reply to                    

D. answer to

已知在一个样本中,50个数据分别落在5个组内,第一、二、三、五组数据个数分别为2、8、15、5,第四组的频数和频率分别是___________________.