在彩色电视机中普遍采用（）电源。
A.直流
B.串联型稳压
C.开关稳压
D.串联型稳压电源或开关稳压
参考答案：C
已知P（-3，m）和Q（1，m）是抛物线y=2x2+bx+1上的两点．
（1）求b的值；
（2）判断关于x的一元二次方程2x2+bx+1=0是否有实数根，若有，求出它的实数根；若没有，请说明理由；
（3）将抛物线y=2x2+bx+1的图象向上平移k（k是正整数）个单位，使平移后的图象与x轴无交点，求k的最小值．
Which of the following statements about duration is FALSE （）
A. There is a direct relationship between yield to maturity and duration.
B. There is an inverse relationship between coupon and duration.
C. There is a direct relationship between duration and maturity.
