Why are airports, trains, cruises , and bus stations often considered the highest targets for biological warfare?

a combination of large groups of people, and they are all travelling- which increases the spread.

The reason airports, trains, cruises, and bus stations are considered high risk areas for biological warfare is because they all have a high number of people in them which means that there will be more fatalities.

they are known to reach a speed of 21 miles per hour

igneous rocks are formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  

the composition depends on the source magma. there are almost a thousand types of composition found till now.  

hope this