题干
-- Mum, can I go to the zoo with Jack?
-- When your homework , you can.
A：is done
B：was done
C：has been done
答案(点此获取答案解析）
C
-- Mum, can I go to the zoo with Jack?
-- When your homework , you can.
A：is done
B：was done
C：has been done
O2（g）＝H2O（g） △H1
H2（g）＝NH3（g） △H3 
O2（g）＝2NO2（g）＋3H2O（g）的△H为
