原生演替的顶级是森林。 [ ]
A. 正确
B. 错误
What is the main idea of the passage？
A. Scientific appraisal shows that multifactorial diseases are often due in part to environmental conditions.
B. In the past half century, scientists have been able to find only a few principles that apply to all diseases.
C. Each disease has single underlying mechanism that must be understood before the disease can be cured.
D. Many diseases that were once life-threatening can now be cured by antibiotics.
The author's purpose in writing this passage is to______.
A. argue for teaching bright and not-so-bright pupils in the same class
B. recommend pair work and group work for classroom activities
C. offer advice on the proper use of the library
D. emphasize the importance of appropriate formal classroom teaching
Which of the following diseases is caused by a vitamin deficiency？
A. Lobar pneumonia.
B. Tertiary syphilis.
Chronic tuberculosis.
D. Pernicious anemia.
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