Here are the best answers to complete the sentences based on the context provided:

1. **C. however** - This option effectively contrasts the fact that NYC has a large subway system with the reality of traffic being bad.

2. **A. empty** - This word conveys that the roads would be less filled with cars if more people used bicycles.

3. **C. for** - This preposition indicates that the bicycles are made available for anyone to use.

4. **D. another** - This word suggests that a person can ride a bicycle from one bike rack to another.

5. **A. who** - This relative pronoun refers to people that do not own bikes.

So the completed sentences would look like this:

New York City, which has a population of nearly 9 million, is one of the densest cities in the world. **However**, it has the world's largest subway system, traffic can still be quite bad, particularly at rush hour. The city decided that it would be a good idea to encourage more people to use bicycles. If more people rode bicycles, the roads would be less **empty** with cars. Also, when you ride a bicycle, you are exercising, which makes you healthy. But how can you encourage people to ride more bikes?

The city came up with an innovative solution. In 2013, city workers began installing long racks of bicycles in different neighborhoods. These bicycles were, for a small fee, available **for** anyone to use. A person could ride the bicycles from one bike rack to **another** bike rack and park it there. This system was ideal for people **who** did not own bikes and wanted to take a bicycle on a short ride without having to return it to the place they took it from. This also made it possible to move quickly between areas that did not connect easily by the subway.