$9.$ When balancing a crankshaft, which factor must be considered to avoid resonant conditions at operating speed?

$-$ A$)$ Thermal expansion

$-$ B$)$ Material fatigue limits

$-$ C$)$ Critical speed

$-$ D$)$ Oil film thickness

$10.$ In multi$-$cylinder engines, how does altering the crankpin arrangement affect the engine's balance?

$-$ A$)$ Changes the amplitude of torsional vibrations

$-$ B$)$ Affects the overall mass distribution along the crankshaft

$-$ C$)$ Modifies the engine's thermal characteristics

$-$ D$)$ Alters the air intake efficiency