在20℃时煤（包括煤的孔隙）的质量称之（） ["块密度","真密度","视密度"]
要声明一个长度为256个字符的定长字符串变量str，下列语句正确的是（） ["dim str as string","dim str as string（256）","dim str as string[256]","dim str as string*256"]
以下不是电子商务采用的安全技术的是（）。 ["软件动态跟踪技术","数字签名","加密技术","安全电子交易规范"]
When a marine surveyor is employed to assist in the move of a mobile offshore drilling unit，he（）. ["becomes responsible for the safe movement of the rig","relieves the Offshore Installation Manager at the moment the rig is free from the bottom","must be designated as the person in charge of the rig","is a specialist who is hired to provide advice and guidance on aspects of the move"]
飞行中出现危险品事故着陆后，以下哪项措施是错误的？（） ["地面人员打开货舱门之前应让旅客下飞机","打开货舱门之前应让机组下飞机","打开货舱门之前旅客和机组保留在飞机上","向地面工作人员指明危险物品及其存放地点"]
简述茎双穴吸虫病的症状。
