Many things must be done before a shuttle can launch. The shuttle has many parts. They are made in different places in the United States. The finished parts are sent to NASA's Kennedy Space Center in Florida. The pieces of the shuttle go to the Vehicle Assembly Building. NASA workers connect all the parts. NASA calls it "stacking."
After all of the parts are put together, the whole vehicle is called the Space Transportation System. Now, it's time to move to the launch pad.
The Space Transportation System rides on a crawler. The top of the crawler is as big as a baseball diamond. The crawler has tracks of wheels like a tank. It is very, very slow. It moves about one mile per hour on a road called the crawlerway.
When the shuttle gets to the launch pad, NASA has more work to do before countdown. The shuttle may sit on the launch pad for weeks. This is the countdown clock. It tracks the amount of time until launch.
The astronauts get into the orbiter about 3 hours before launch. Finally, everyone waits for the words, "3 . . . 2 . . . 1 . . . Liftoff!"
http://www.nasa.gov/audience/forkids/home

What is this passage's organizational structure?
A.
problem and solution
B.
chronological order
C.
compare and contrast
D.
cause and effect