Translate the following sentence into English.
儿女们不明白，市场上蔬菜如此便宜，老人干嘛还找麻烦，花力气自己种菜。
【正确答案】：
THE OLD MAN'S CHILDREN DON'T UNDERSTAND WHY HE TAKES SO MUCH TROUBLE TO GROW VEGETABLES SINCE THEY ARE SO INEXPENSIVE ON THE MARKET.
【题目解析】：
考点：句子主干为“儿女们不明白”，后接why引导的从句。找麻烦take much trouble to do sth.；便宜inexpensive／cheap。