What time do you usually _____ in the morning?          [ ]

A. have a breakfast                      

B. have breakfast          

C. have the breakfast                  

D. have breakfasts

一对夫妇所生育的子女中，性状上差异很多，这种差异主要来自（）

A.基因重组

B.基因突变

C.环境影响

D.疾病影响