X、Y、Z、W均为短周期元素．它们在周期表中相对位置如图所示，若Y原子的最外层电子数是内层电子数的3倍，下列说法中不正确的是（　　）

A．原子半径：W＞Z＞Y＞X

B．气态氢化物的稳定性：Y＞Z

C．最高价氧化物对应水化物的酸性：W＞Z

D．四种元素的单质中，Z的熔沸点最高

One ofthe main resposibilities of ICMP is to report(1).Although technology has produced increasingly relaible transmission media, errors still exist and must be handled. IP is an (2) 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 (3) errors, it simply report them. Error correction is left to the higher-level potocols. Error messages are always sent to the original (4) because the only information available in the datagram about the route is the source and destination IP addresses. ICMP use the source IP address to sent the error massage to the source of the (5).

空白（2）处应选择（）

A.reliable

B.unreliable

C.available

D.unavailable