Find the period of small oscillations in a vertical plane performed by a ball of mass `m=40g` fixed at the middle of a horizontallly stretched string `l=1.0m` in length. The tension of the string is assumed to be constant and equal to `F=10N`.
A. `0.1s`
B. `0.2s`
C. `0.3s`
D. `0.4s`