What is the Jeans instability?

A When a growing protoplanet becomes too large, it is more likely to be split apart by subsequent bombardment than to grow further.

B If a solar nebula is too massive it will continue to collapse to become a black hole and will not form planets.

C If the disk of matter surrounding a solar nebula becomes too hot, it will dissipate, and planets will not form.

D If a cloud of gas and dust is sufficiently dense, it will collapse due to its own gravity.

Atrain is traveling at 30.0 m/s relative to the ground in still air. the frequency of the note emitted by the train whistle is 262 hz. the speed of sound in air should be taken as 344 m/s. what frequency (f approach) is heard by a passenger on a train moving at a speed of 18.0 m/s relative to the ground in a direction opposite to the first train and approaching it? what frequency (f recede) is heard by a passenger on a train moving at a speed of 18.0 m/s relative to the ground in a direction opposite to the first train and receding from it?