Which two statements about the diffserv model are true？（） ["a class can be inentified as a single application .multiple applications with similar service nees.or be based on the source or destination ip addresses.","a drawback of the diffserv model is that it does not provide backward compatibility with devices that can only use the tos field.","diffserv uses the diffserv field in the MAC header to mark frames into behavior aggregates（BAs）","the diffserv field occupies the same eight bits of the mac header that were previously used for the tos field.","the first six high-order bits of the diffserv field are used to identify the resource reservation protocol（RSVP） value .","the primary goal of diffserv is scalabilty."]
铜元素的存在有助于提高带钢的（）。
当驳岸为土岸时，应选用同一树种、同一规格等距离配置。
如何预防肉鸡胸囊肿和腹水症？
下列关于栅栏门的叙述错误的一项是（）。 ["在进口处应有引导栅栏，站台应有防滑措施；","栅栏门应向内外两个方向开；","栅栏门的内侧应设立止推块；","栅栏门开启方向应与乘客行进方向一致。"]
一个四岁大的男孩在幼儿园里总打别的小朋友。这种现象的出现，行为主义学者和精神分析学者会有什么不同的看法？
