边长为4的正三角形的高为（　　）

方程mx+ny=6的两个解是$\left\{\begin{array}{c}x=1\\ y=1\end{array}\right.$，$\left\{\begin{array}{c}x=2\\ y=-1\end{array}\right.$，则m，n的值分别是（　　）

沧州海兴县位于河北省东南部，渤海之滨，拥有盐碱地约30万亩。在当地“种一瓢收一斗。的盐碱地或中低产田上，中科院农业研究所经过不断改良与实验，使小麦平均每亩产量达到600斤。农业研究所主要通过咸水结冰等方法改造盐碱土壤（传统盐碱地治理方法主要是通过强灌强排、抽取地下淡水降低地下水位，以淡水洗盐，使土壤脱盐），打造“良田”，通过微咸水灌溉等方法进行田间管理，并选育更耐盐碱的作物品种。据此完成下列各题。

阅读下面材料, 在空白处填入适当的内容(一个单词)或括号内单词的正确形式。将答案填写在答题卷的相应位置。

    Everyone seems____(have) his or her own holiday. Dad has Father's Day. Mom has Mother's Day. Children have Children's Day. ____(lover)have Valentine's Day. But is there a special day____older people?

Of course. The Chongyang Festival is the holiday when Chinese people show love for their____（old）. Celebrated on the ninth day of the ninth lunar month, the festival is also known as the Double Ninth Festival. Nine is the____(high) digit(数字), so it____(regard)that the two together mean a long life.

The festival comes ____the golden time of autumn. The clear weather and the joy of harvest make for a happy atmosphere. On the day, people____(traditional)climb a mountain and carry dogwood(茱萸). People in ancient times believed____the plant could drive away evil____(spirit) and prevent people against cold in winter.