Answer:
Option C
Explanation:
Key idea Velocity ofa rocket at any time t.
v= u\left(\frac{m_{0}}{m}\right) -gt
where, u= speed of exhausted gases
m_{0}= initial mass of the rocket
and m= mass of the rocket at time t
Given, fuel burned rate (\frac{dm}{dt}) =100 kg/s, u=5 km/s
Then, v= 5ln( \frac{m_{0}}{m})-gt
As, it is given that the gravitational force is ignored and mass of rocket is reduced to \frac{1}{20} th of
it's initial mass i.e, m= \frac{1}{20} m_{0}
or \frac{m_{0}}{m}=20
so, v=5 ln(20) km/s
Hence, the correct option is (c)