What is the least number of weighings needed to determine the sum of weights of 13 watermelons such that exactly two watermelons should be weighed in each weigh?<spanclass=′latex−bold′>a)</span>7<spanclass=′latex−bold′>b)</span>8<spanclass=′latex−bold′>c)</span>9<spanclass=′latex−bold′>d)</span>10<spanclass=′latex−bold′>e)</span>11