Answer:
A
Explanation: A is a example of Air resistance force, which is a contact force.
When sitting down there is gravity , when sat down the chair is pushing back at a equal but opposite force
Answer:
The sound intensity of train is 1000 times greater than that of the library.
Explanation:
We have expression for sound intensity level,

A train whistle has a sound intensity level of 70 dB
We have

A library has a sound intensity level of about 40 dB
We also have

Dividing both equations

The sound intensity of train is 1000 times greater than that of the library.
The scientists describe them as D. Elliptical