《小儿病源方论》提出了养子十法，包括（）

A．一要背暖

B．二要肚暖

C．三要足暖

D．四要头凉

E．五要手凉

The days _____ they spent their childhood together have already gone.            [ ]

A. when                    

B. which

C. that                      

D. where