题目内容：
He cannot be tired _______ he has walked only half a mile. (A) ：since
(B) ：whether
(C) ：when
(D) ：as long as
参考答案：
选择题：He cannot be tired _______ he has walked only half
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(A) ：since
(B) ：whether
(C) ：when
(D) ：as long as