— I saw Helen in the school just now.
— It ______ be Helen. She has gone to the shopping center with her sister.
A:needn't
B:shouldn't
C:can't
D:mustn't
前些天,我场遭遇了低温雨雪天气,某些地区的树枝上甚至出现了“雾凇”。“雾凇”和霜的形成过程很相似,都是 ( )