Q. Operation of helical gears is silent and quiet while the operation of the spur gears is noisy because

• (A) helical gears have some helix angle but the spur gears are straight
• (B) there is end thrust in helical gears
• (C) in helical gears, contact begins at one end of the entering tooth and gradually extends along a diagonal line across the tooth face as the gears rotate
• (D) none of the above