three influenza type A virus subtypes(子类型) have been found in pigs.
A. By far
B. As far as
C. So far
D. Till far
网友回答
A. By far
B. As far as
(答案→)C. So far
D. Till far
解析：C考查far短语辨析。句意：迄今为止，在猪身上共发现3种A型流感病毒的子类型。by far最----，-------得多，用在形容词和副词后；as/so far as 就---而言，远到------；so far 迄今为止，主句常用现在完成时，分析have been found完成时被动语态可见so far符合句意和语言用法。