Don't throw waste paper everywhere.It's our duty to keep our school .
A:clean
B:cleaned
C:cleaning
D:cleans
— .
下列关于病毒的叙述,不正确的是( )
)如图,偶函数f(x)的图象形如字母M(图1),奇函数g(x)的图象形如字母N(图2),若方程f(g(x))=0.g(f(x))=0的实根个数分别为a,b,则a+b=( )