Have you decided ——————for Canada?A,when you will leave B.when you leaving选A是的,那能不能选B?

Have you decided ——————for Canada?A,when you will leave B.when you leaving
选A是的,那能不能选B?

decide后面接的是一个宾语从句,既然是一个句子,就必须有谓语,如果选B的话,谓语就没理由,如果是When you are leaving 的话,就对了

如果选B，从句缺少谓语动词be , When you are leaving 或 When you leave

have you decided 后跟when 引导的宾语从句， 从句要有谓语， will leave 作谓语。 而 leaving 不能作谓语 ， 如果改成 you are leaving 就正确了。
所以正确答案为A。

Have you decided when you will leave for Canada？
答案选A，宾语从句后面必须是一句完整的句子，而B选项不是一句完整的句子