B because 2800 divide by 40 is 20
Answer:
Velocity, v = 0.239 m/s
Explanation:
Given that,
The distance between two consecutive nodes of a standing wave is 20.9 cm = 0.209 m
The hand generating the pulses moves up and down through a complete cycle 2.57 times every 4.47 s.
For a standing wave, the distance between two consecutive nodes is equal to half of the wavelength.

Frequency is number of cycles per unit time.

Now we can find the velocity of the wave.
Velocity = frequency × wavelength
v = 0.574 × 0.418
v = 0.239 m/s
So, the velocity of the wave is 0.239 m/s.
Answer: drying towels at the beach.
Explanation:
Radiation simply has to do with the energy that is gotten from a particular source and then goes through some materials. It simply means the way energy is being transmitted as waves or heat through a certain medium.
From the options given, the scenario whereby a radiation takes place is when drying towels on the beach.
Rocket fuel will and smoke will emit from the thrusters.