A block of mass 0.1 kg is connected to an elastic spring of spring constant 640 Nm−1 and oscillates in a medium of constant 10−2 kg s−1. The system dissipates its energy gradually. The time taken for its mechanical energy of vibration to drop to half of its initial value, is closest to:
a. 2 s
b. 3.5 s
c. 5 s
d. 7 s