脐带一般长为( )。
A．20cm B．30～40cm C．50～60cmD．70cm E．80～100cm
参考答案：C
解析： 脐带
三个连续奇数，中间一个为2n-1，则这三个连续奇数之和为______．
Mr Zhang left for America in 1998, and he didn't return home 2002. [ ]
A. unless
B. when
C. until
D. since
