如图，点A、B在直线l上，AB=10cm，⊙B的半径为1cm，点C在直线l上，过点C作直线CD且∠DCB=30°，直线CD从A点出发以每秒4cm的速度自左向右平行运动，与此同时，⊙B的半径也不断增大，其半径r（cm）与时间t（秒）之间的关系式为r=1+t（t≥0），当直线CD出发 ____秒直线CD恰好与⊙B相切．

阅读下面短文, 从短文后各题所给的四个选项(A、B、C、D)中, 选出可以填入空白处的最佳选项。
    Many people enjoy taking trips by themselves. Unluckily, they do not always have the best1.Instead, people should consider group tours. Taking a group tour is a wise choice for people to2themselves.
    First, group tours plan travelers' routes, so people don't have to3where they are going next or how they are getting there. They can just4and enjoy their trip.5, my family once took a group tour to China. The travel agency (旅行社)6everything for us. We just looked at the schedule the travel agency had7and followed it. Thanks to the travel agency, we had a 8trip. This made things 9on my father since he didn't have to worry about where we would go next and 10at night．
    Another11of group tours is that they allow people to communicate with one another while traveling．Travelers are able to develop12with people they would never have met13they are traveling together for a number of days. As I know, my parents 14Russia on a group tour for their honeymoon(蜜月). They15another couple on that trip, and everyone had a great time together. Even after many years, they still16together from time to time. If my parents had traveled by themselves, it would have been17for them to get to know this couple.
    In short，there is 18better than a group tour when traveling. Group tours schedule everything on the trip, and they give us the19to meet others and make friends with them. Group tours are the 20way to travel when taking a trip．

如图分别是蜗牛、麻雀、青娃、蝴蝶、鲤鱼5种动物，请用所学知识分析回答：

如图所示，分别是音叉和钢琴发出的两列声波在同一示波器上显示的波形，则这两列声波（  ）

高尔基体具有极性，靠近细胞核的一面称为形成面，接近细胞膜的一面称为成熟面．从形成面到成熟面，膜的厚度和化学成分逐渐发生改变．下列叙述正确的是（    ）