Sound waves must have intensity if a person is to hear them.
a. true
b. false

a. True

Explanation:

Intensity is what we commonly call Volume, sound waves with no intensity or volume wouldn´t be hearable by humans, so the statement would be True, Sound waves must have intensity if a person is to hear them. The most usual unit to measure sound intencity is Decibels, and  a person with good ears can hear up to -15 dB.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.