Question

a simple ______________ is a connected set of rotating gears that transmits power from an input to an output. driving gear gear train driven gear gear ratio idler gears

Explanation:

A gear train is a set of gears that transmit power. The question describes a connected set of rotating gears for power transmission. "Driving Gear" is a single gear, "Driven Gear" is also a single gear, "Gear Ratio" is a ratio (not a set), and "Idler Gears" are just part of a gear train. So, "Gear Train" fits the description.

Answer:

Gear Train