晶粒边界熔化，是体积形缺陷，存在于焊缝或热影响区。轻度过烧呈小黑灰斑点，重度过烧呈黑色蜂窝状。
参考答案：对
--- __________ are all these together?
--- 425 yuan.
A．How much
B．How many
C．How long
D．How often
