Michelle wants to use a model to demonstrate that water flows faster through layers of large pebbles than through layers of sand. She builds her model by attaching a fine screen to one end of a pipe and filling half the pipe with a mixture of sand and large pebbles. She then pours water through the pipe and records how long it takes all the water to flow through the pipe.

Knowing what Michelle wants to demonstrate, identify the problem in her model's design.
A.
The pipe is only half full—it should be completely filled with the mixture of sand and pebbles.
B.
There is not enough variety in sediment size—sediment of other sizes should be mixed with the sand and pebbles.
C.
The order of events is wrong—the sand and pebbles should be placed in the pipe after the water flows through.
D.
The sand and pebbles are mixed together—water flow through each type of sediment should be demonstrated separately.