Ans. is `a' i.e. 10th day of shoest menstural cycle o Shoest period gives information about the first day of the feile period. The shoest cycle minus 18 days gives first day of feile period. For example, in a cylce of 28 days (most common), first day of feile period will be 10th day of cycle (28 -18 =10). o Longest period gives information about the last day of the feile period. The longest cycle minus 10 days gives the last day of feile period. For example, in a cycle of 28 days, last day of feile period will be 18th day of cycle (28 - 10 =18).