第18题: [单项选择]What conditions are needed for shrubs to become established A. Ferns must take root. B. The ground must be covered with grass. C. More soil must accumulate. D. Smaller plants must die out. 参考答案:C 答案解析:[解析]事实细节题。在第三段的中间,只有在长过苔藓之后草木和shrub(灌木)才能生长,因为苔藓已经固定了相当一部分土壤,有利于植物生长,故选C)。A)项“蕨类植物必须先扎根”与B)项“地面上必须覆盖