68. You should           before you start an aerobic exercise program.

A. make sure that you are young enough

B. make sure that you have learned warm-up exercises

C. go to a doctor for a medical examination

D. learn a course of aerobic exercises on TV

69. What should your pulse rate be at the beginning of the cool-down period?

A. Around 70 beats.              B. Around 90 beats.

C. Around 130 beats.    D. Around 150 beats.

70. The warm-up period should last at least         .

A. 5 minutes   B. 10 minutes    C. 15 minutes    D. 20 minutes

第二节 (共5小题，每小题2分，满分10分)

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

Trees are one of the oldest living things on earth .They play a very important role for human’s existence. Trees are important for life as they provide food and oxygen. The importance of tree continues to increase and more benefits of trees are being discovered as their role expands to satisfy the needs created by our modern lifestyles. The forests and trees that are planted by humans work together with us to make the world look better and more beautiful. __71____Some of them are as follows.

Trees can clean soil and control soil erosion. Poisonous chemicals and pollutants from various factories enter into the soil, thus causing soil pollution.___72____Soil erosion is a serious problem that causes degradation(退化)of soil and leads to floods. Trees and plant roots bind the soil and thus reduce soil erosion. Also they conserve rainwater and reduce sediment(沉积)deposits.

Trees produce oxygen . Trees play an important role in reducing the greenhouse gases. They consume carbon dioxide and produce oxygen.___73____A fully grown tree produces oxygen which is sufficient for10 people for a complete year. Also, they act as filters of the air that we breathe.

Trees are of commercial importance. Trees fulfill the need of fuel for cooking in many areas around the world. Trees also provide timber for furniture and hundreds of household items. Wood pulp is used for making paper in industries.____74____Some trees are even used to make medicines.

____75____They provide shelter and food for a number of animal species across the globe. Trees are especially important as they form green corridors connecting habitats with each other and with the countryside beyond.