Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

An Binh used to be a small village on the bank of the Red River. Local people mainly worked on farms and grew their own food. Public transport was not very convenient at that time. Villagers just went around the local area by bicycle or on foot. The only way for them to travel to another town or city was to cross the river by boat, then get on the train.

Their life, however, started to change from the year 2000. A bridge across the Red River was built, connecting the village with one of the biggest motorways in the country. Since then, many local people have migrated to big cities. The village has also attracted people to come and start a business here. In addition, new industrial zone has been developed. Local people and residents of neighbouring villages have started working in the industrial zone's factories. The infrastructure has also been improved. Clean water facilities and drainage systems have been installed. A number of high-rise buildings have been buitt. Supermarkets, hospitals, and many other facilities have also been provided.

The village has now grown into an urbanised town with a population of over 20,000 people. Life has become easier for its residents. However, there are now some new problems. A large number of young people are unemployed because they lack the necessary knowledge and skills to work in the factories. In addition, the town is getting more and more crowded. As a result, pollution has increased and has sparked concern among local residents and the authorities. Despite al these concerns, the life of An Bin's residents has generally become much better.

While we ______ in the park, Mary fell over.