题目内容：

Have you ever wondered why the roots of the plants always know which way to grow—into the soil but not above it? Some British scientists have recently solved this mystery.

It turns out that roots have special hairs that tie them into the soil and help them grow their way past obstacles, a team at the John Inners Center in Norwich reports in the February 29 issue of Journal Science.

“The key is in the fuzzy(有绒毛的)coat of hairs on the roots of plants,” says professor Liam Dolan. “We have found a growth control system that enables these hairs to find their way and to become longer when their path is clear.”

Root hairs explore the soil in much the same way a person would feel their way in the dark. If they come across an obstacle, they make their way around until they can continue growing in an opening. In the meantime, the plant is held in place as the hairs grip(紧紧抓住)the soil.

The hairs are guided by a clever chemical trick. A protein(蛋白质) at the tip of the root hairs called RHD2 helps them to take calcium(钙) from the soil. Calcium makes the hairs grow, and produce more RHD2, and take more calcium.

But when an obstacle blocks the hair’s path, or the hair reaches the surface of the soil, the cycle is broken and growth starts in another direction.

This system gives plants the flexibility to explore a complex environment and to live in even the most unpromising soils, says Dolan.

In poor soils such as in parts of Australia and Africa, native plants have adapted by producing enormous numbers of root hairs. A better understanding of this adaptation will allow scientists to develop hairy rooted crops that can grow in unfriendly environments.

According to Dolan, “Research in the John Inners Center is taking a breeding approach to increase hair length in wheat but it will be some time before new cultivars(栽培变种) are developed.”

1.The passage is mainly about _______.

A．why the roots of plants grow into the soil but not above it

B．how the roots of plants grow into the soil but not above it

C．the process of plants growth

D．a growth control system of plants

2.How does the circle work?

A．RHD2 takes calcium from the soil, which makes hairs grow, and produce RHD2 and take more calcium

B．Roots take RHD2 and calcium from the soil and produce RHD2

C．Roots make hairs grow, and produce RHD2, and take more calcium

D．RHD2 takes protein, which makes hairs grow, and produce RHD2, and take more calcium

3.We can infer from the last paragraph that ________.

A．hairy rooted crops can grow better in unfriendly environments

B．a new cultivar of wheat with long hair has been developed by scientists

C．a new cultivar of wheat with long hair is still on test

D．the roots of plants always know which way to grow