题目内容：

Exposed soil isn’t part of nature’s master plan. Good examples where soil is naturally found with no plants growing in are beneath freshly removed trees, or where the ground has been burnt by a land fire. In these situations bare soil isn’t bare for long; within days seedlings (芽) begin to appear and cover the ground. In a few months’ time, the scar is hardly noticeable.

Unlike these natural examples, ploughed (耕犁) fields and freshly dug gardens are obvious examples created by man. But, left to nature, even these bare soils soon turn green with a large number of tiny seedlings. In fields, gardens and on grasslands, roadside and sports fields, along streets and on paths and pavements, we are constantly battling to stop weeds from growing. All we are doing is stop the recovering powers of nature.

Whenever soil is exposed, weeds act like a kind of medicine to reduce the potentially harmful rays of the sun, so the sooner plant growth covers the soil over again, the better.

Weeds also help to improve the fertility (肥沃) of the soil. Their roots get the soil together, improving its structure and creating a more stable environment in which soil life can grow fast. Those weeds with a deep root draw up plant nutrients from deeper in the ground, making them available to plants growing near the soil surface. Above ground, the stems (茎) of weeds help trap fallen leaves which break down into the soil, adding to the fertility of the soil.

As the soil becomes more fertile, different kinds of plants start to replace the “pioneering” weeds. Bushes move in to take the place by gradually shading them out, followed in turn by trees, which eventually push up through the bushes, finally shading them out too. Fallen leaves from the bushes and trees carry on the job of building soil fertility that was begun by those very first weeds that grew on the bare soil. Then, when one of these mature trees is removed in a storm, leaving a wound of bare soil in the earth, the whole process starts again.

1.When will soil be left bare?

A. A land fire has just happened.    B. A young tree is planted in.

C. No one ploughs the land.    D. No crops are planted in.

2.Which of the following prevents the bare soil from recovering?

A. Seedlings appearing in gardens.

B. Reducing harmful rays of the sun.

C. Removing weeds from pavements.

D. Replacing plants in sports fields.

3.What can weeds do for the soil?

A. They create soil life.

B. They help improve the soil.

C. They get nutrients from the sun.

D. They break down fallen leaves.

4.How can the soil remain fertile after trees move in?

A. By adding more weeds to the soil.

B. By taking out the weeds constantly.

C. By removing the previous bushes.

D. By making use of the fallen leaves.