—There were already five in the car, but they managed to take me as well.—It ( ) a comfortable journey.
couldn't have been这个答案是错的吧

—There were already five in the car, but they managed to take me as well.—It ( ) a comfortable journey.
couldn't have been这个答案是对的。意思是“一定不/不可能”，本来要用mustn‘t have been，但must不能用于否定句和疑问句。若在否定句和疑问句中要用can/could 代替must。
eg：
It must be him. 一定是他。
It can’t be him。不可能是他。
He must have arrived very early. 他一定到得很早。
He couldn‘t have arrived very early. 他不可能到得很早。