简述杂种猫的概念。
参考答案:由不同品种的猫交配而成,染色体上的等位基因不一样。
The police found that the house____and a lot of things___. [ ]
A. has broken into; has been stolen
B. had broken into; had been stolen
C. has been broken into; stolen
D. had been broken into; stolen
乒乓球比赛规则规定:一局比赛,双方比分在10平前,一方连续发球2次后,对方再连续发球2次,依次轮换,每次发球,胜方得1分,负方得0分。设在甲、乙的比赛中,每次发球,发球方得1分的概率为0.6,各次发球的胜负结果相互独立。甲、乙的一局比赛中,甲先发球。
(1)求开始第4次发球时,甲、乙的比分为1比2的概率;
(2)ξ表示开始第4次发球时乙的得分,求ξ的期望。