You're Waiting to Cross a Busy City Street as a City

You're waiting to cross a busy city street as a city bus pulls up to the curb. As the driver stops the bus, the brakes squeal loudly. A few people get on, and then the bus pulls away from the curb. As the bus accelerates, the engine labors into gear and exhaust fumes spew out the back. The high-pitched squeal of the brakes was caused by _____ sound waves, and the equally loud but low-pitched roar of the engine was caused by _____ sound waves.

A) low-frequency; high-frequency
B) high-frequency; low-frequency
C) low-amplitude; high-amplitude
D) high-amplitude; low-amplitude