You adjust the frequency of the rf oscillator on the nmr electronics until there is negligible beating between the oscillator and the spin precessions, yielding equal frequencies of f= 15.30 mhz. the static magnetic field from the rare-earth magnet is:

whichever one it is, it's definitely not one that you included on the list you provided.

i think the answer is a, hope this : )

force tends to be low and long diustance masses are high

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me