儿女们不明白，市场上蔬菜如此便宜，老人干嘛还自找麻烦，花力气自己种菜。
The old man s children don t understand why he takes (so much) trouble to grow vegetables since (or, when) they are so inexpensive (or, cheap) on the market.
儿女们不明白，市场上蔬菜如此便宜，老人干嘛还自找麻烦，花力气自己种菜。
The old man s children don t understand why he takes (so much) trouble to grow vegetables since (or, when) they are so inexpensive (or, cheap) on the market.