Which statement is true? select one of the options below as your a. quarks always occur in groups with leptons. b. quarks always occur independently. c. quarks always occur in groups with other quarks. d. quarks always occur in groups with bosons. e. quarks always occur in groups with electrons.

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?