在边长为1的正三角形ABC的边AB、AC上分别取D、E两点，使沿线段DE折叠三角形时，顶点A正好落在边BC上。AD的长度的最小值为        。

Bears ______ fat stores throughout the summer and fall to have energy enough to last them through winter sleep.

A．build up

B．pack up

C．bring up

D．take up