Our English ______ far better in a month if we try harder.
A:becomes
B:will become
C:has become
D:became
计算:5a3b2c÷10a2bc=____ .
图(a)为某同学组装完成的简易多用电表的电路图.图中E是电池;R1、R2、R3、R4和R5是固定电阻,R6是可变电阻;表头G的满偏电流为250 μA,内阻为480Ω.虚线方框内为换挡开关,A端和B端分别于两表笔相连.该多用电表有5个挡位,5个挡位为:直流电压1V挡和5V挡,直流电流1mA挡和2.5mA挡,欧姆×100Ω挡.
当二次函数y=x2+4x+9取最小值时,x的值为