To understand why the maximum allowed blood alcohol concentration for a driver with two years of experience with the AM license who has just obtained the B license is **0.25 milligrams per liter of exhaled air**, it is important to consider the following:

- **Driving licenses**: The AM license allows the driving of mopeds and light vehicles, while the B license allows the driving of cars that can carry up to nine passengers, among other vehicles. Both licenses have different requirements and authorizations.

- **Allowed blood alcohol concentration**: The allowed blood alcohol concentration varies according to the type of driver and vehicle. For drivers who have obtained their license less than two years ago, the allowed concentration is **0.15 milligrams per liter of air**. However, for any other driver and vehicle, the allowed concentration is **0.25 milligrams per liter of air**.

- **Driver's situation**: In this case, the driver has two years of experience with the AM license and has just obtained the B license. This means that they are no longer considered a novice driver under the B license, as their previous experience with the AM license counts towards the total driving time. Therefore, the standard blood alcohol concentration of **0.25 milligrams per liter of air** applies to drivers with more than two years of experience.

In summary, since the driver has more than two years of total experience with driving licenses, the maximum allowed blood alcohol concentration is **0.25 milligrams per liter of exhaled air**.