I think that his main reason for rejecting the possibility is that he could not imagine a process w（）it could happen.（请根据字母填入正确的单词；填写答案时需填入完整的单词。）

I think that his main reason for rejecting the possibility is that he could not imagine a process w（）it could happen.（请根据字母填入正确的单词；填写答案时需填入完整的单词。）
正确答案：whereby