A company plans to develop a prototype-weeding machine that uses cutting blades with optical sensors and microprocessors that distinguish weeds from crop plants by differences in shade of color.  The inventor of the machine claims that it will reduce labor costs by virtually eliminating the need for manual weeding.

Which of the following is a consideration in favor of the company’s implementing its plan to develop the prototype?

1. There is a considerable degree of variation in shade of color between weeds of different species.

2. The shade of color of some plants tends to change appreciably over the course of their growing season.

3. When crops are weeded manually, overall size and leaf shape are taken into account in distinguishing crop plants from weeds.

4. Selection and genetic manipulation allow plants of virtually any species to be economically bred to have a distinctive shade of color without altering their other characteristics.

5. Farm laborers who are responsible for the manual weeding of crops carry out other agricultural duties at times in the growing season when extensive weeding is not necessary.

Why D? not A?

ls的仔细认真的翻译下a选项，然后应该就知道为什么不选a了

Especially，please translate "variation" exactly.

我给你个明确的解答吧

种子颜色从黑色到白色变化，并且是一系列紧密的色系分布，也就是说无法明确区分颜色的分别。但是这样的颜色分布也可以说成是a considerable degree of variation in shade of color 。

不知道这样解释是否清楚。