为什么会出现这种错误：
ex071001.obj : error LNK2001: unresolved external symbol "struct StudNode * head" (?head@@3PAUStudNode@@A)
ex071002.obj : error LNK2001: unresolved external symbol "struct StudNode * head" (?head@@3PAUStudNode@@A)
ex071003.obj : error LNK2001: unresolved external symbol "struct StudNode * head" (?head@@3PAUStudNode@@A)
ex071002.obj : error LNK2001: unresolved external symbol "struct StudNode * thisN" (?thisN@@3PAUStudNode@@A)
ex071003.obj : error LNK2001: unresolved external symbol "struct StudNode * thisN" (?thisN@@3PAUStudNode@@A)
ex071002.obj : error LNK2001: unresolved external symbol "struct StudNode * newN" (?newN@@3PAUStudNode@@A)
Debug/lianbiao.exe : fatal error LNK1120: 3 unresolved externals
执行 link.exe 时出错.