●One of the main resposibilities of ICMP is to report（）.Although technology has produced increasingly reliable transmission media, errors still exist and must be handled．IP is an （） protocol．This means that error checking and error control are not a concern of IP．ICMP was designed，in part，to compensate for this shortcoming．Howerver, ICMP does not（） errors，it simply reports them．Error correction is left to the higher-level potocols．Error messages are always sent to the original （） because the only information available in the datagram about the route is the source and destination IP addresses．ICMP uses the source IP address to send the error massage to the source of the （） .

IP is an （） protocol

A．reliable

B．unreliable

C．available

D．unavailable

班主任通过查阅学生的作业、学籍卡片等来了解学生的方法属于（）

A.分析书面材料法

B.问卷法

C.调查访问法

D.观察法