Pathogenic means bacterium, virus, or other micro organism that causes disease. So the answer is C.
Answer:
1) Recursive definition: 
2) At the beginning of the 4th interval
Explanation:
1)
The initial population of the bacteria at time zero is

Here we are told that the reading is taken every two hours; we call this time interval "n", so

And also, after every time interval n, the number of bacteria has tripled.
This means that when n = 1,

And when n=2,

Applied recursively, we get

And substituting p0,
(1)
2)
Here we want to find at the beginning of which interval there are

bacteria.
This means that we can rewrite eq.(1) as

By simplifying,

Which means that

However, this means that the number of bacteria is 1,350,000 after 3 time intervals; therefore, at the beginning of the 4th interval.
<span> Negative staining method is used to view viruses, bacteria, bacterial flagella, biological membrane structures, lipid aggregates and proteins or protein aggregates, which all have a low electron-scattering power. This method is used to view cells or certain organelles of cells more clearly under the microscope in the field of microbiology. </span>
Http://www.livescience.com/22537-skeletal-system.html
Answer:
Layer A is the crust and Layer B is the mantle.