A 500kg rocket is set for verticle firing. The exhaust speed is 800`ms^-2` . To give an initial upward acceleration of 20`ms^-2` , the amount of gas ejected per second to supply the needed thrust will be (g=10 `ms^-2` )
A. `127.5 kg//s`
B. `187.5 kg//s`
C. `85 kg//s`
D. `137.5 kg//s`