Cars are lots of fun, but they could also be dangerous. We have to be careful when we drive them or ride in them.

It's always a good idea to put on your seat belt when you're in a car. Why? Think about this example: You put an egg on a skateboard and give it a push. If the skateboard hits a stone, it will stop, but the egg won't. It will fly through the air, hit the ground and break.

Now, think what would happen if you tied the egg to the skateboard. When the skateboard hits a stone, the egg won't go flying; it will stay safely on the skateboard.

Volvo, a famous Swedish carmaker, was the first to use seat belts in 1849.

Air bags are also very important for car safety, because sometimes a seat belt isn't enough. If the car is going really fast and runs into something, seat belts could even hurt the people who wear them. Most new cars have air bags in front of and next to the seats. When a car hits something, its air bags will come out quickly in less than one second to keep the people inside safe.

小题1:The passage mainly tells us                  .

A．two important ways to keep safe in cars

B．how air bags work to make cars safe

C．how seat belts work to make cars safe

D．why we must tie the egg to the skateboard小题2:The writer gives the example of the egg to _.

A．tell us that eggs are easy to break

B．show how to wear a seat belt

C．show why it's important to wear a seat belt in a car

D．tell us what a skateboard is小题3:Seat belts can make us safer because .

A．we are interested in them

B．they can stop us from hitting other cars

C．they can help us stay safely on the seats

D．they are made of strong materials小题4:Air bags are important for cars because _.

A．they are made of plastic

B．they can also help us to keep safe in a car

C．they can keep cars running slowly

D．they are put in front of and next to the seats小题5:Which of the following is TRUE according to this passage?

A．It won't be dangerous if there are seat belts or air bags in a car.

B．Now most of new cars still have only one air bag.

C．When a car hits something, its air bags will come out a few minutes later.

D．Volvo was the first to use seat belts.

某大型工程项目由政府投资建设，业主委托某招标代理公司代理施工招标。招标代理公、司确定该项目采用公开招标方式招标，招标公告在当地政府规定的招标信息网上发布。招标文件中规定：投标担保可采用投标保证金或投标保函方式担保；评标方法采用经评审的最低投标价法；投标有效期为60天。
业主对招标代理公司提出以下要求：为了避免潜在的投标人过多，项目招标公告只在本市日报上发布，且采用邀请招标方式招标。
项目施工招标信息发布以后，共有12家潜在的投标人报名参加投标。业主认为报名参加投标的人数太多，为减少评标工作量，要求招标代理公司仅对报名的潜在投标人的资质条件、业绩进行资格审查。
开标后发现：
A投标人投标报价为8000万元，为最低投标价，经评审后推荐其为中标候选人。
B投标人在开标后又提交了一份补充说明，提出可以降价5%。
C投标人提交的银行投标保函有效期为70天。
D投标人投标文件的投标函盖有企业及企业法定代表人的印章，但没有加盖项目负责人的印章。
E投标人与其他投标人组成了联合体投标，附有各方资质证书，但没有联合体共同投标协议书。
F投标人的投标报价最高，故F投标人在开标后第二天撤回了其投标文件。
经过对投标文件的评审，A投标人被确定为中标候选人。发出中标通知书后，招标人和A投标人进行合同谈判，希望A投标人能再压缩工期、降低费用。经谈判后双方达成一致：不压缩工期，降价3%。

分析A、B、C、D、E投标人的投标文件是否有效如果无效，请说明理由。