对具有线性相关关系的变量x，y有一组观测数据(x_i，y_i)( i=1，2，…，8)，其回归直线方程是$\stackrel{\wedge }{y}=\frac{1}{3}x+a$：，且x₁+x₂+x₃+…+x₈=2(y₁+y₂+y₃+…+y₈)=6，则实数a的值是( )

根据所学的相关知识，回答下列有关植物激素的问题：

下列关于生态系统的说法正确的是(     )

阅读理解，选择正确的答案。

    April Fools'Day is a day to play jokes on others．No one knows how it began，but people think it first began in France．

    Each country celebrates April Fools'Day differently．In France，people call the April fools "April Fish"．They tape a paper fish to their friends' backs to fool them．When he or she finds this，they shout” April Fish!”

    In the UK，people play jokes only in the morning．You are a“noodle”if someone fools you．

    In the US，people play small jokes on their friends on the first of April．They may point down to your shoe and say，“Your shoelace iS untied．”If you believe them and look down，you are an April fool then．