如图所示,一块钉板上水平方向和垂直方向相邻两钉的距离都是一个单位,用橡皮筋构成如图的一个四边形,那么这个四边形的面积为( )
A:2.5
B:5
C:7.5
D:9
The farmer is very busy because he's so _______ sheep to keep and so ______ work to do every day.
已知等差数列an满足a2+a4=4,a3+a5=10,则它的前10项的和S10=( )