根据短文内容，从短文后的选项中选出能填入空白处的最佳选项，选项中有两项为多余选项。
The world's first underground(地铁) was built in London in 1863，At the time，the government was looking for a way to reduce traffic problems.There were so many people living in the poor areas of the city.__1__ If people had an easy way to get to and from work, they would live outside of the city and this would help reduce traffic problems in London.As a result, the idea of the London Underground was born.
The plans for building the London Underground met with several problems. __2_ A steam train pulled the cars along the first underground track which was six kilometers long.__3__ They were treated to comfortable seats inside the cars. However, the smoke from the engine(发动机) soon filled the air in the tunnels(隧道). _4__ Even with its problems, riding the underground did become popular.It carried nine million riders in its first year.
Modern underground has come a long way since the early days of the London Underground. __5_ Shopping malls built in traffic stations have helped the popularity of the underground.Nowadays many stations include underground department stores, supermarkets and restaurants.Shoppers never have to go above ground!
A．But the first track was finally opened in January,1863.
B．It is not easy to build the underground.
C．About 30,000 people got on the underground the first day.
D．Sometimes there are not enough seats for the passengers during rush hours.
E．Fans had to be put in the tunnels later to keep the air clean enough for people to breathe.
F．Today there are more than one hundred underground systems around the world.
G．So the officials decided to make it possible for workers to live outside of London and travel easily to work each day.
1G 2A 3C 4E 5F
, because it often ____________.
shan is very tall.