$2.$The modulus of rigidity is crucial for calculating:

A$)$ A spring's color change under stress

B$)$ The electrical resistance of a spring

C$)$ The torsional stress in helical torsion springs

D$)$ The thermal conductivity of a spring

$3.$Leaf springs are characterized by:

A$)$ Their ability to change color with temperature

B$)$ The use of multiple thin layers to absorb shock

C$)$ Emitting light when subjected to stress

D$)$ Conducting electricity more efficiently than coil springs