You have been collecting measurements of DNA concentrations in soil, trying to estimate the average concentration of DNA

[answerhappygod]

You have been collecting measurements of DNA concentrations in
soil, trying to estimate the average concentration of DNA in soil.
You have collected 10 samples so far, and you currently calculated
the standard error of your concentration to be 0.1 μg/mL. However,
you want to be more certain of the true value.
How many total samples do you need to collect (including the
ones you already have) to reduce the size of your standard error to
1/3th of its current value?
Select one:
a. 30 samples
b. 90 samples
c. 40 samples
d. 270 samples