Electrons have a mass of about . A. 2,000 times that of a neutron

B. 1/2,000 that of a proton

C. the same as a neutron

D. the same as a proton

B. 1/2,000 that of a proton

Explanation:

the mass of an electron is 9.1093837015 × 10−31 kg, this in only 1/2,000 that of a proton.

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

answerdmitry mendeleyev made a workable classification of the elements