姜片虫病主要的传染源是()
A.猪
B.犬
C.猫
D.生植物
E.患者
参考答案:A, E
Because of the financial crisis, days are gone ______ local 5-star hotels charged 6,000 yuan for one night. [ ]
A. if
B. when
C. which
D. since
观察下列各式:
(x-1)(x+1)=x2-1;
(x-1)(x2+x+1)=x3-1;
(x-1)(x3+x2+x+1)=x4-1,
根据前面各式的规律计算(x-1)(xn+xn-1+… +x+1)。