纤芯的折射率分布近似为抛物线型，为（）光纤。
A.多模
B.单模
C.阶跃
D.渐变
参考答案：D
胃溃疡腹痛发生时间（）
A．空腹痛
B．进食后30～60分钟
C．进餐后3～4小时
D．餐前1小时
E．餐前半小时
The（1） is a general description of the architecture of a workflow management system used by the WFMC, in which the main components and the associated interfaces are summarized. The workflow enactment service is the heart of a workflow system which consists of several（2）.
空白（1）处应选择（）
A.waterfall model
B.workflowreference model
C.evolutionary model
D.spiral model
