解答词汇题的三个步骤

18. They took ____ measures to prevent poisonous gases from escaping. A) fruitful B) beneficial C) valid D) effective
19. The teacher ____ his lesson about France with photographs of the people who live there. A) indicate B) conducted C) instructed D) illustrated
20. Children in schools are ____ into grades, according to how much they know. A) qualified B) identified C) clarified D) classified