An experiment is designed to determine if the amount of sugar in a drink affects the likelihood that bees will be attracted to it. sample a is 100 ml of pure water. sample b is 100 ml of water with 1 gram of sugar dissolved in it. sample c is 100 ml of water with 2 grams of sugar dissolved in it.

all samples are placed on a picnic table. one observer is assigned to each drink. the observers count the number of bees that visit the drink during a 15-minute interval.

in the above experiment, which factor is the independent variable?

the amount of sugar in the sample
the color of the sample
the temperature of the sample
the number of bees that visit