科学家成功地把人的抗病毒干扰素基因植入烟草细胞中的DNA分子上，使烟草获得了抗病毒的能力．这个事实说明，DNA上控制生物性状的基本单位是（　　）

A．基因

B．糖类

C．脂肪

D．蛋白质

Directions:

Write an essay of 160--200 words based on the following drawing. In your essay, you should

1) describe the picture briefly,

2) explain its intended meaning, and then

3) support your view with an example/examples.

You should write neatly on ANSWER SHEET 2.