单选题 Which of the following most logically completes the argument below?
Each year every employee of SAI Corporation must enroll in one of the two health insurance plans offered by SAI. One plan requires a sizable monetary contribution from employees; the other plan is paid for entirely by SAI. Many SAI employees enroll in the plan requiring employee contributions. This fact does not show that they feel that this plan's benefits are superior to those provided by the plan requiring no employee contribution since______.

A、 the plan that requires an employee contribution costs and enrolled employee significantly less per year than do typical health insurance plans offered by corporations other than SAI
B、 only SAI employees who have worked for SAI for at least fifteen years are eligible to enroll in the plan paid for entirely by SAI
C、 the two health insurance plans currently offered by SAI are substantially the same plans SAI has offered for the past ten years
D、 most of the SAI employees enrolled in the plan paid for entirely by SAI are under 50 years old
E、 (E) both plans offered by SAI provide benefits not only for employees of SAI but also for children and spouses of enrolled employees

【正确答案】 B

【答案解析】[解析] 本题为完成段落题型的带since、because的解释题，即解释since之前的那一句话“为什么自己支付一部分钱的计划并不好于完全由SAI付款的计划，但许多雇员仍参加前一计划”。(B)说明完全由SAI支付的计划只对为SAI工作了15年的雇员有效，并不是每一个人都可以参加的，解释了since之前的那一句话，因此(B)正确。