下列说法正确的个数有（   ）

①﹣0.5x²y³与5y²x³是同类项；

②2π与﹣4不是同类项；

③两个单项式的和一定是多项式；

④单项式mn³的系数与次数之和为4．

已知x,y满足线性约束条件$\left\{\begin{array}{c}\begin{array}{c}x-y+1\ge 0\\ x+y-2\le 0\\ x+4y+1\ge 0\end{array}\\ \end{array}\right.$，若$\overrightarrow{a}=\left(x,-2\right)$，$\overrightarrow{b}=\left(1,y\right)$，则$z=$$\overrightarrow{a}·\overrightarrow{b}$的最大值是（   ）

综合填空。阅读短文，根据短文内容及首字母提示，在空白处填入一个适当的单词。

    Years ago in Scotland, the Clark family had a dream. The couple worked and saved, making p____ for their nine children and themselves to travel to the USA. It had taken years before they saved enough money and got everything r____ for the trip.

    The whole family was excited about their new life. However, seven days b____ they left for the USA, the youngest son was bitten by a dog. The doctor treated the boy, b____ because there was a possibility of rabies（狂犬病）, they would be kept away from others for fourteen days.

    The family's dream could not come t____. They would not be able to make the trip to America as they had planned. Watching the ship leave—without the Clark family, the father was quite a____ with both his son and God for their bad luck.

    Five days later, a piece of sad news s____ throughout Scotland—the strong and powerful ship, Titanic, had gone down, taking h____ of lives with it. The Clark family was to have been on that ship, but because the son was bitten by a dog, they were left b____ in Scotland.

    When Mr. Clark heard the news, he thanked his son for saving the family. He thanked the God for t____ a sad thing into a good one.

    Although we may not always understand, all things happen for a reason.