Which of the following conditions is sufficient to conclude that revolving a planar region about the x-axis will produce cross sections that are washers rather than solid disks?
a) The region is bounded below by the x-axis and above by a single non-negative function throughout the interval.
b) Every vertical line through the region intersects two distinct boundary curves before meeting the x-axis.
c) Every horizontal line through the region intersects two distinct boundary curves before meeting the x-axis.
d) The region touches the x-axis at least one point within the interval of revolution.