He is a top student but he ________exercises. [ ]
A.usually
B.always
C.seldom
D.often
答案:C
已知数列{xn}满足xn+3=xn,xn+2=|xn+1-xn|(n∈N*),若x1=1,x2=a(a≤1,a≠0)则数列{xn}的前2010项的和S2010为( )
A.1340
B.1338
C.670
D.669
甲肝病毒是怎样进入人体的?