对以自然选择学说为核心的现代生物进化理论,正确的理解是①环境改变使生物产生适应性的变异 ②隔离是新物种形成的必要条件 ③变异是不定向的 ④变异是定向的 ⑤自然选择使种群的基因频率定向改变并决定生物进化的方向
A ______ works in a bookstore. A. writer B. bookseller C. reader
A ______ can help you find the way. A. dancer B. doctor C. policeman
Can you ______? Yes, I can. A. swim B. swims C. swimming
Listen! The girls ______ in the classroom now. A. sing B. sings C. are singing
______ dress is it? It’s Amy’s. A. What B. Which C. Whose