A mass is attached to the end of a spring that is then stretched past its equilibrium point and released, causing it to oscillate with a period of 0.4 s. If the spring constant k is 140 N/m, what is the magnitude of the mass attached to the spring?
a) 0.4 kg
b) 0.6 kg
c) 0.8 kg
d) 1.2 kg