一个生态演替的顶级群落的典型特征是（）。 ["生态系统相对稳定","生物量的增长达到最大","植物与动物的物种数量持续快速增长","每一年的生态系统净生产量都不同"]
In a hydraulic system, （） are used to protect the system from over pressure. ["stop valves","check valves","safety valves","reducing valve"]
The burden of proving unseaworthiness is upon those who（）it. ["approves","alleges","allows","permit"]
关于RJ-45接线的连接标准有（）。 ["B568A和S568B","S568A和T568B","N568A和S568B","T568A和T568B"]
A hydraulic motor may be driven by （）. ["an electric motor","a diesel engine","a compressor","oil"]
渠道控制的基础是（）。
