they discovered many animals could be people's friends改为被动语态 lt 空空that many animals could
问题描述：
they discovered many animals could be people's friends改为被动语态 lt 空空that many animals could
be people's friends
the river because polluted in 2005改为现在完成时态
the river 空空polluted 空six years
答
they discovered many animals could be people's friends
改为被动语态 lt (was discovered) that many animals could be people's friends
the river because(是became吧) polluted in 2005
改为现在完成时态 the river(had been )polluted (for) six years