下列各句中修辞手法使用不恰当的一句是[ ]

A．树林是一片绿色的海洋，轻风是海洋的呼吸。    

B．珍珠是贝痛苦的结晶，是海的泪。    

C．面对风暴的欺凌，松柏挺胸不屈，杨柳弯腰逢迎，江河寂然无语，高山昂然抗争。

D．太阳从地平线上露出笑脸，用她那柔美的金色手指，悄悄地捡走了草地上的珠玑。

About fifty years ago, plant physiologists set out to grow roots by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and the usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along fine on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require energy, of course. The process of respiration uses sugar to make the high energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.The study of isolated roots has provided an understanding of the relationship between shoots and roots in intact plants. The leaves of the shoots provide the roots with sugar and vitamins, and the roots provide the shoots with water and minerals. In addition, roots can provide the shoots with organic nitrogen compounds. This comes in handy for the growth of buds in the early spring when leaves are not yet functioning. Once leaves begin photosynthesizing, they produce protein, but only mature leaves can "export" protein to the rest of the plant in the form of amino acids.

The word "themselves" in Paragraph 1 refers to________.

A．plant physiologists

B．solutions

C．laboratory flasks

D．roots