Https://youtu. be/1B4guKd-mPQ​

a. a primary source combines several different accounts of an event. b. a primary source generally benefits from having been written after an event.

explanation:

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.