— They speak perfect English. How long have they learnt it?

— ______ they were seven years old.

A．Since

B．When

C．After

D．Until

做心电图检查示室早二联律，则该心律失常最可能的原因是

A．心肌缺血

B．心力衰竭

C．电解质紊乱

D．洋地黄中毒

E．感染