It was only after she had read the papers _______ Mary realized the homework before her was extremely difficult to complete.

A．when

B．that

C．which

D．what

若吸入盐酸：迅速脱离现场至空气新鲜处，保持呼吸道通畅；如呼吸困难，给输氧；如呼吸停止，立即进行人工呼吸就医。