Audible beats are formed bu the interference of two waves
a. of slightly different frequencies
b. of greatly different frequencies
c. with equal frequencies, but traveling in opposite directions
d. from the same vibrating source.

Audible beats are formed by the interferences of two waves in which are from the slightly different frequencies which is letter a. It is because having two waves that consist of slightly different frequency causes or produce the audible beats to be formed in which they are beats that causes sound to be either loud or soft.

Julia would be observing a. static electricity was caused by her sweater rubbing against her shirt and transferring electrons. she didn't touch anything metal, so that would cancel out b.she most likely didn't rub her sweater against her hair either, so that cancels out c.she also didn't scoot across the carpet while trying to take her sweater off, that cancels out d.so that leaves a.a. is correct.

The answer is c hope this