please help mee its a rlly hard test I will give brainliest to the person that answers right.

1. Frequency times wavelength always equals
a. 1
b. amplitude
c. speed
d. 1/speed

2. A node is caused by
a. constructive interference
b. destructive interference
c. reflection
d. refraction

3. The speed of sound in steel is greater than it is in air because
steel is
a. more elastic than air
b. less elastic than air
c. more dense than air
d. less dense than air

4. For a wave of wavelength 4 m and frequency 8 waves/sec.,
a. amplitude = 32 m.
b. amplitude = 0.5 m.
c. speed =32 m/sec.
d. speed = 2 m/sec.