We know she had to tap twice, because Hetty had to put out her candles and throw off her black lace scarf; but after the second tap the door was opened immediately.

我们知道她必须敲两下， 因为海蒂必须熄灭蜡烛并甩掉她的黑色蕾丝围巾； 但第二次敲门后， 门立即打开了。