A person ties a rock to a string and whirls it around in a circle such that sometimes the rock is going straight upward and sometimes the rock is going straight down (the axis of the circle is horizontal). She whirls the rock at the minimum speed (constant in time) such that the string is always taut (no sag). If she were to use a longer string, she would have to whirl the rock at a _____.
a. lower velocity.
b. the same velocity.
c. higher velocity.
