As a train rushes away from a crossing, the pitch of its horn goes down because . a sound waves get farther apart as the source of the sound moves closer
b sound waves get farther apart as the source of the sound moves away
c sound waves get closer together as the source of the sound moves away
d sound waves stay the same distance from each other as the source of the sound moves away

The awnser is b

Explanation:

The sound waves get smaller as the sound moves farther away, and since the sound waves are smaller, the sound is quieter. This is known as the Doppler effect.

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6

i just learnt this recently but i need to look through my notes again