焊接符号有哪些含义？
参考答案：
表示工件四周都需焊接；6表示焊缝高度为6mm；△表示角焊缝。
________ many times，I still didn't know how to operate the machine.
A．Having taught
B．Having been taught
C．Being taught
D．Teaching
核能有何特点是什么？
