What element is lowest density of all of the elements?

Hypothesis: the smell of decomposing apples may attract fruit flies more than the smell of decomposing bananas.

Experiment: two separated fruit fly populations with a sample size statistically significant (N: 500 or higher) must be grown in clean boxes. Subsequently, the fruit fly populations should be simultaneously released in a tube that connects the two boxes with other two boxes containing: -Box 1: apples in decomposition and -Box 2: bananas in decomposition. After one hour, the fruit boxes must be simultaneously closed and the fruit flies must be counted. Finally, it is necessary to apply a statistical index in order to calculate the significance between groups (i.e., fruit flies closed in both fruit boxes).

Positive control: fruit flies released and counted in boxes containing apples and bananas in normal conditions.  

Negative control: fruit flies released and counted in clean boxes.