（），称为锅炉的水循环。
参考答案：水和汽水混合物在锅炉蒸发受热面回路中的循环流动过程
The figure above is a regular hexagon with center H, The shaded area is a parallelogram that shares three vertices with the hexagon; its fourth vertex is the center of the hexagon. If the length of one side of the hexagon is 8 centimeters, what is the area of the unshaded region ？（）
A.A
B.B
C.C
D.D
E.E
台阶朝向采空区一侧的倾斜面称为台阶坡面。
