如果选择了一个前面的历史记录，所有位于其后的历史记录都无效或变成了灰色显示，这说明了什么

下列哪种格式存储可以包含Alpha通道

Directions:

Inthissectionyouwillheararecordedshortpassage.Thepassageisprintedinthetestpaper,butwithsomewordsorphrasesmissing.Thepassagewillbereadtwotimes.YouarerequiredtoputthemissingwordsorphrasesontheAnswerSheetinorderofthenumberedblanksaccordingtowhatyouhear.Ithinkwe'llmbeginnow.FirstI'dliketowelcomeyouallandthankyouforyourcoming,especiallyatsuchshortnotice.Iknowyouareallverybusyandit'sdifficulttotaketimeawayfromyour______formeetings.Asyoucanseeontheagenda,todaywewillfocusontheupcoming______.Firstwe'lldiscussthegroupsthatwillbecominginfromGermany.Afterthat,we'lldiscusstheNorthAmericanTours,______bytheAsiantours.Iftime______,wewillalsodiscusstheAustraliantourswhicharebookedforearlySeptember.Finally,I'mgoingtorequestsomefeedbackfromallofyou______lastyear'stoursandwhereyouthinkwecanimprove.

混凝土防渗墙施工中，混凝土的浇筑在清孔换浆后（）小时内必须开始。

砖砌体水平缝的砂浆饱满度应不低于（）。