In the past few years, research has suggested that Vitamin D may increase the level of serotonin(血清素), one of the key nurotransmitters influencing our mood, and ________it helps to relieve mood disorders.
.—Who______ it be that is knocking at the door? —It ______ be Father, but I’m not sure.
.. The day________ he was looking forward________ in the end.
..---Who ________ it be that is knocking at the door?---It________ be Mother, but I’m not sure.
..If you cheat in the exam, you could hardly________ it.
..He once studied in Zhejiang University, ________he went abroad for further study.