Ball A is thrown vertically downward from the top of a 100m tower with a speed of 10m/s. One second later, ball B is projected vertically upward from the ground with a speed of 40m/s. Neglect air resistance and take g=10m/s2. At what height above the ground do the two balls meet?
a) 35m
b) 60m
c) 47m
d) 70m