1.when are you flying to Beijing?可不可以用when will you fly to Beijing?请说理由
2. A:that boy must be li Lei.
B:he can't Li Lei . Li Lei went to his hometown three days ago.
可不可以用 A:that boy can be li Lei.请说理由
must be 一定是 can be 可能是
下句B:he can't Li Lei . Li Lei went to his hometown three days ago.不是说明上句可能是吗