According to the information in the third paragraph, which of the foll

Accordingtotheinformationinthethirdparagraph,whichofthefollowingimprovementsmadeitpossibletolengthenthetoneproducedbythepiano?（）

Theintr oductionofpedals

Theuseofheavywires

Theuseoffelt-paddedhammerheads

Themetalframeconstr uction

答案及解析：事实细节题。全文最后一句话中“theintr oductionofpedalstosustaintone”表明了踏板的使用使得钢琴的音色延长。