One ofthe main resposibilities ofICMP is to report  （） . Although technology hasproduced increasingly relaible transmission media, errors still exist and must be handled. IP isan  （）  protocol. This means that error checking and error control are not a concem ofIP. ICMP was designed, in part, to compensate for this shortcoming. Howerver, ICMP doesnot  （） errors, it simply report them. Error correction is left to the higher-levelpotocols. Error messages are always sent to the original  （）  because the only informationavailable in the datagram about the route is the source and destination IP addresses. ICMP usethe source IP address to sent the error massage to the source ofthe（）.

Error messages are always sent to the original（）because the only informationavailable in the datagram about the route is the source and destination IP addresses.

A．user

B．destination

C．source

D．program

园林工人为了提高移栽树木的成活率，树木的根部要带一坨土，这是为了保护好树木的______，移栽时树根部插有一根塑料管与外界相通，这有利于根的______作用，树栽好后还要剪掉一部分枝叶并遮阳，这可降低植物的______作用；菜农常采用白天适当增强光照，夜晚适当降低室温的措施，以提高蔬菜产量．其中的道理是增加光照可使______增强，降低温度可使______减弱，从而增加有机物的积累．温室内培育蔬菜还可以通过增加空气中______浓度从而达到增产．大棚南瓜开花的季节，有经验的瓜农会将租来的蜜蜂放养在自己的大棚内，这样做的目的是______．