In a simple pendulum experiment for determination of acceleration due to gravity ( g ), time taken for 20 oscillations is measured by using a watch of 1 second least count. The mean value of time taken comes out to be 30 s. The length of pendulum is measured by using a meter scale of least count 1 mm and the value obtained is 55.0 cm. The percentage error in the determination of g is close to:
a. 0.7%
b. 0.2%
c. 3.5%
d. 6.8%