A boy's catapult is made of rubber cord which is 42 cm long, with 6 mm diameter of cross-section and of negligible mass. The boy keeps a stone weighing 0.02 kg on it and stretches the cord by 20 cm by applying a constant force. When released, the stone flies off with a velocity of 20 ms−1. Neglect the change in the area of cross-section of the cord while stretched. The Young's modulus of rubber is closest to:
a. 106 N/m2
b. 104 N/m2
c. 108 N/m2
d. 103 N/m2