属于缺陷病毒的是()
属于DNA病毒的是()
属于小RNA病毒科的是()
其基因组最小的是()
输血后肝炎的主要病原体是()

A．HAV

B．HBV

C．HCV

D．HDV

E．HEV

Few students can understand the sentence until it ________ twice or three times.[ ]

A. explains

B. is explained

C. will be explained

D. has explained