某单位某月1~12日安排甲、乙、丙三个值夜班，每人值班4天。三人各自值班期数字之和相等。已知甲头两天值夜班，乙9、10日值夜班，问丙在自己第一天与最后一天值夜班之间，最多有几天不用值夜班？

A.6 

B.4 

C.2 

D.0

Potato cyst nematodes are a pest of potato crops. The nematodes can lie dormant for several years in their cysts, which are protective capsules, and do not emerge except in the presence of chemicals emitted by potato roots. A company that has identified the relevant chemicals is planning to market them to potato farmers to spread on their fields when no potatoes are planted; any nematodes that emerge will soon starve to death.
Which of the following, if true, best supports the claim that the company’s plan will be successful

A．Nematodes that have emerged from their cysts can be killed by ordinary pesticides.

B．The only part of a potato plant that a nematode eats is the roots.

C．Some bacteria commonly present in the roots of potatoes digest the chemicals that cause the nematodes to emerge from their cysts.

D．Trials have shown that spreading even minute quantities of the chemicals on potato fields caused nine-tenths of the nematodes present to emerge from their cysts.

E．(E) The chemicals that cause the nematodes to emerge from their cysts are not emitted all the time the potato plant is growing.