依据《合同法》的规定，下列文件中，属于要约的是( )。
A．招标公告
B．投标书
C．寄送的价目表
D．招股说明书
参考答案：B
—Where is your English teacher? I’m looking for him everywhere.
—He _____ the Internet in the computer room.
A．will search
B．has searched
C．searched
D．is searching
解释PN节的单向导电性？
