The nurse is caring for a toddler with Down syndrome. To help the toddler cope with painful procedures, the nurse can

A．prepare the child by positive self-talk.

B．establish a time limit to get ready for the procedure.

C．hold and rock him and give him a security object.

D．count and sing with the child.

在平面直角坐标系下，用图解法求解线性规划问题的条件是含有两个或两个以上决策变量的线性规划。