if a population of 50 paramoecium present in a pool increases to 150 after an hour what would be the growth rate of population?

A. 50 per hour
B.200 per hour
C.5 p hour
D. 100 p h

an answer was posted before( which was 2) but wasnt part of the above options. 

This is a question direct from NCERT EXAMPLER
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kindly help me with this question with proper steps  

Thanking you :)