假设在一个由草、鹿、狼组成的相对封闭的生态系统中，把狼杀绝，鹿群的数量将会（  ）

70－20=____

8＋60=____

35－5=____

30＋30=____

电阻R₁、R₂串联接在电路中，R₁两端的电压为3伏，R₂两端的电压为9伏，R₁+R₂=600欧，求：

根据下列短文，选择最佳选项。

    Sam likes soccer. He always watches soccer games on TV. His wife (妻子) Nina likes watching TV plays (电视剧). One evening at 7:00, Sam watches an interesting soccer game on TV. The TV also has a good TV play. It is Nina's favorite. She wants to watch it very much, but Sam sits before (在……之前) the TV. He doesn't let Nina watch the TV play. Nina is not happy, so she goes to her mother's home at 7:30 in the evening. When she comes into her mother's home, her father watches that soccer game, too.     

     “Where is my mother, Dad?”asks Nina.

     “She goes to your grandma's home,”says her father.

     “Why?”

     “I want to watch the soccer game, but she likes to watch a TV play.”