这辆山地自行车有21个齿轮，可在各种地形上平稳骑行。

This mountain bike has 21 ___gears for smooth riding on various terrains.

它有24个主动齿轮和多连杆凯夫拉刹车，以提升性能。

It has 24 active ___gears and multilink Kevlar brakes for enhanced performance.

骑自行车上山时应适当换挡，以避免过度用力。

You should shift ___gears appropriately when cycling uphill to avoid strain.