若α∈$\left(0,\frac{\mathrm{\pi }}{2}\right)$，且$\cos \left(\alpha +\frac{\mathrm{\pi }}{6}\right)=-\frac{\sqrt{2}}{4}$，则cosα=____ 

根据短文理解，从A、B、C、D四个选项中选择最佳一项填空。

　　How do you imagine the future? Will life in the future be better, worse or the same as now? 1 do you think of the future?

People 2 that life will probably be very different in 2050. First of all, planes will be very 3, so flying will be very cheap. Everyone will have a small car. Fuel will be new and clean, but expensive. We will 4 robots everywhere. And space planes will take us around the world 5 two hours. Homes will be warm in winter and cool in summer, so they will be very comfortable. We will use the sun 6 our homes. Computers will be very small so they will be very 7. People will do many things on the Internet.

At that time, water will become one of our most8 problems. In many places, farmers will be in great need of water to9 fruit and vegetables.

    By 2050, we will be able to help blind and deaf people see again and10 again. At that time, our life will be better and better.

取一条较长的软绳，用手握住一端（O点）连续上下抖动，在绳上形成一列简谐横波．已知O点完成一次全振动所用的时间为T．某一时刻的波形如图所示，绳上a、b两点均处于平衡位置．下列说法正确的是（   ）