a÷b×$\frac{1}{b}$÷c×$\frac{1}{c}$÷d×$\frac{1}{d}$等于（　　）

将下列单词或词组填入空格，每空格限填一词，每个单词或词组只能填一次。

    Although we still don't know whether using mobile phones can cause cancer, we should try to use our phones ____.Here are some tips:

    *Use a headset（耳机） or speaker.

    Headsets give out much less radiation than phones.Some wireless headsets give out low-level radiation. However, you should ____ your headset when you're not talking to anyone.Using a phone with a speaker will also make it safer.

    *Listen more, talk less. Your phone gives out radiation when you talk or text, but not when you're listening or getting messages.

    *Hold the phone away from your body when you are talking, not ____ your ear. Also, don't keep it in your pocket.

    *Choosing texting over talking. Phones give out less radiation when sending texts than when making calls.Texting also keeps radiation away from your head.

    * Poor signal? Stay off the phone. Fewer signal bars（信号数） on your phone ____ that the phone needs to give out more radiation to work. Make and take calls when your phone has a strong signal.

已知$-\frac{\mathrm{\pi }}{2}<\alpha <0$，则点$P\left(\sin \alpha ,\cos \alpha \right)$位于（   ）

阅读司马光的《宋太祖善待赵普》 一文，完成题。
         太祖时，赵韩王普^①为宰相，车驾因出，忽幸其第。时两浙王钱俶^②方遣使致书及海物十瓶于韩王，置在左庑^③下。会车驾至，仓卒出迎，不及屏^④也。上顾见，问何物，韩王以实对。上曰：“此海物必佳。”即命启之，皆满贮瓜子金也。韩王皇恐，顿首谢曰：“臣未发书，实不知，若知之，当奏闻而却之。”上笑曰：“但取之，无虑。彼谓国家事皆由汝书生耳。”因命韩王谢而受之。韩王东京宅皆用此金所修也。
        【注释】①赵韩王普，赵普死后曾被追封为韩王。② 钱俶，吴越王，北宋太平兴国三年时纳土归宋，治理两浙。③庑，堂下周围的走廊。④屏，退避。