The human body contains about 10 trillion (10^13) bacteria and approximately 20,000 genes. To find how many times the number of bacteria is greater than the number of genes, you can divide the number of bacteria by the number of genes:
\[ \frac{10^{13} \text{ bacteria}}{20,000 \text{ genes}} = \frac{10^{13}}{2 \times 10^4} = \frac{10^{13}}{2 \times 10^4} = 5 \times 10^{8} \]
This indicates that the number of bacteria is about 500,000,000 times greater than the number of genes.
Therefore, the correct answer based on the closest match from your options would be:
D: 400,000,000 (as it is the closest estimation).