Why is ultrasound imaging considered safer than X-ray imaging for monitoring the development of a fetus during pregnancy?

In a longitudinal sound wave, a compression is a region of high pressure, while a rarefaction is a region of __________ pressure.

Which of the following best describes the physical structure of an electromagnetic wave as it propagates through space?

If a wave has a frequency of $25\ \text{Hz}$, how many seconds does it take for one complete wave cycle to pass a fixed point?

What generally happens to the upper limit of the human hearing range (typically $20\ \text{kHz}$) as a person gets older?