幼儿急疹最主要的临床特征是
A．发热出疹
B．热退疹出
C．为红色斑丘疹
D．皮疹1日内出齐
E．疹退后不脱屑无色素沉着
参考答案：B
—Li Ping went to Australia to study and travel last year.
—Yes, I know. He told me he would never forget his meaningful there.
A．explorations
B．expressions
C．experiences
D．Experiments
一平面谐波的表达式为y=0.002cos（400πt-20πx）（SI），取k=0，±1，±2，…，则t=1s时各波谷所在的位置为：（）
A.A
B.B
C.C
D.D
