闭合性肾损伤最常用的治疗方法是（）。

A.肾脏手术检查

B.放导尿管膀胱引流

C.肾脏引流

D.严密观察，暂保守治疗

E.肾脏裂伤缝合

I have been trying my best to study English ________ I want to have a chance to be a volunteer for the Beijing Olympics.       [ ]

A.until  

B. because  

C. though    

D. when