Help i give brainilest Which is the function of the exoskeleton in animals? (2 points)

a. Produces the sperm used in reproduction
b. Protects against different weather
Protects against their environment's (this answer is incorrect so don't choose it stoopid)
d. Works with muscles to allow movement

Engines for propeller-driven aircraft are limited in their maximum rotational speed by the fact that the tip speed of the propeller must not approach the speed of sound in air (mach i). taking 6 ft as a typical diameter for a propeller of a light airplane and 1100 fils as the speed of sound, find the upper limit on the rpm (revolutions per minute) of the propeller shaft.