Mrs. Green doesn't like sports. She has much work to do at home. Her mother is coming this afternoon and she goes to meet her. Near the school she looks at her watch. She has some time. So she goes into the school. Her son isn't in the classroom. She asks a boy and knows they're behind the school. She sees some boys playing there. Her son is there too. She watches them playing and soon she's angry. She takes her son to the wall. “Go home with me, Richard.” She says, "Don't play with them. I can buy a nice ball for you. Then you don't need to scramble for the ball."

人体健康与食品、药物等关系密切．有些学生从小偏食，不爱吃水果、蔬菜，结果营养缺乏、发育不良，这主要是由于摄取下列哪种物质不足引起的．（　　）