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
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