抛物线y=x²在点M$(\frac{1}{2},\frac{1}{4})$处的切线的倾斜角是（   ）

下图是某农产品的变化曲线图，P和Q分别表示该农产品的价格和数量，需求曲线D到D₁移动表示农产品的需求变化趋势，供给曲线S到S₁移动表示该农产品供给变化趋势，E到E1表示农产品的均衡价格变化趋势，一般情况下，可以推断出引起下图变化趋势的原因可能有（  ）。

①产销的中间流通环节降低成本                  

②越来越多农民外出务工

③城镇人口和总人口持续增加                    

④该农产品种植面积扩大

增加：在缺词处加一个漏字符号（八），并在其下面写出该加的词。

删除：把多余的词用斜线（\）划掉。

修改：在错的词下画一横线，并在该词下面写出修改后的词。

注意：1)．每处错误及其修改均仅限一词；

2)．只允许修改10处，多者（从第11处起）不计分。

    We are so happy to have Ms Green here to stay with us today. Now I'll make a brief introduce about her. During the period of 1990 and 1996, she studied at Chicago University in America, learn English and the Chinese language. After graduation, she comes to China and has been teaching English in middle schools.

    Ms Green will teach us English for a few year in our school. While stay here, she will help improve your spoken English. Beside, she will tell us more about the education in Canada.

    I hope Ms Green will have good time in our school.

已知圆C：x²+y²﹣4x=0，l为过点P（3，0）的直线，则（　　）