进行人群高血压普查时应选择的疾病频率测量指标是（）
A.发病率
B.发病专率
C.罹患率
D.时点患病率
E.期间患病率
参考答案：D
The students _ their homework every day. Now they _ their homework. [ ]
A. do; do
B. does; doing
C. doing; are doing
D. do; are doing
运行中发电机频率允许变化范围±0.2Hz，过低的危害是（）。
A.使转子两端的鼓风量减少，温度升高
B.发电机电动势和频率、磁通成正比，为保持电动势不变，必须加大励磁电流，使线圈温度升高
C.可能引起汽轮机叶片共振而断叶片
D.厂用电动机转速降低，电能质量受到影响
