平时我们应该常吃海带等含碘丰富的食物，这对预防什么病有好处（  ）

如图所示，属于增大摩擦的是(    )

①“若 $\overrightarrow{a}\cdot \overrightarrow{b}>0$ ，则 $\overrightarrow{a}$ 与 $\overrightarrow{b}$ 的夹角为锐角”及它的逆命题均为真命题

②若 $p:\forall x\in R,2^x>0$ ,则 $\neg \text{p}:\exists x_0\in R,2^{x_0}<0$

③“ $ab\le 1$ ”是“ $a\le 1$ 或 $b\le 1$ ”的充分不必要条件

④命题“ $△\text{ABC}$ 中，若 $A>B$ ，则 $\sin A>\sin B$ ”的逆命题为真命题

其中正确命题的个数是(    )

阅读短文，回答问题。China is a big country. There is a population of 1.3 billion（13亿人口）in China. People speak Chinese in China. Beijing is the capital ofChina. It`s an old and famous city. It`s in the north .In China, there are manybig cities. Shanghai is a famous city, too. It`s in the east of China. I livein shanghai. My favourite city is Sanya. Sanya is in the south. Lhasa(拉萨)is in the west.