如图，⊙O中，弦AC= $\sqrt{15}$ ，沿AC折叠劣弧 $\widehat{AC}$ 交直径AB于D，DB= $\frac{1}{2}$ ，则直径AB=（   ）

任务型阅读
    There are some easy things you can do to protect the environment and the earth. Choose ideas from the list or come up with a few of your own.
    Plant flowers, grass or trees.
    Whenever you visit a park or a beach, take away what you take there—keep rubbish in a bag until you can put it in a bin.
    Turn off the lights and TV sets when you leave the room. This can save a lot of electricity.
    Turn off the tap （水龙头）when you brush your teeth. You can save some water by not letting it run. Also, use a glass cup instead of a paper cup because this saves paper.
    Keep the doors and windows closed in winter to keep cold air out.
    Give your old clothes to poor chirldren instead of throwing them away.
    Use both sides of the paper.
    Stop pouring dirty water into the rivers or lakes nearby.
Encourage all your friends to do the same things as you do to help protect the earth. You don't have to wait until Earth Day to do these things. Make every day Earth Day. If everyone makes a contribution(贡献) to protecting the environment, the world will become much more beautiful.

阅读下面文字，回答问题。

【甲】答谢中书书

    山川之美，古来共谈。高峰入云，清流见底。两岸石壁，五色交辉。青林翠竹，四时俱备。晓雾将歇，猿鸟乱鸣；夕日欲颓，沉鳞竞跃。实是欲界之仙都。自康乐以来，未复有能与其奇者。

【乙】与朱元思书

    风烟俱净，天山共色。从流飘荡，任意东西。自富阳至桐庐一百许里，奇山异水，天下独绝。

    水皆缥碧，千丈见底。游鱼细石，直视无碍。急湍甚箭，猛浪若奔。

    夹岸高山，皆生寒树，负势竞上，互相轩邈，争高直指，千百成峰。泉水激石，泠泠作响；好鸟相鸣，嘤嘤成韵。蝉则千转不穷，猿则百叫无绝。鸢飞戾天者，望峰息心；经纶世务者，窥谷忘反。横柯上蔽，在昼犹昏；疏条交映，有时见日。

教育部在全国学校开展“中华经典诵读行动”试点，试点学校将在语文、历史、德育等课程中强化经典讲解、诵读内容。此举（     ）

在图中所示的生活和生产实例中，与连通器有关的是（   ）