Itisestimatedthat，currently,about50，000speciesbecome_____everyyear．
heteachertoldthemsincelight________(travel)fasterthansound,lightning________(appear)togobeforethunder.
Thistest________anumberofmultiplechoicequestions.
________,Johnhasbeentoseveralforeigncountries.
Humanbehaviorismostlyaproductoflearning________thebehaviorofanimalsdependsmainlyoninstinct.
