For a van de graaff generator to acquire a charge on its surface what must happen?
a. positive and negative charges must be exactly equal in proportion to each other and remain so for a period of time.
b. an uncharged object must be placed nearby.
c. the metal surface must stay neutral for as long as possible.
d. positive and negative charges must be separated inside the device and one type of charge must be carried upwards to the metal sphere.

The answer is D. A Van de Graaff generator is an electrostatic generator which utilizes a moving belt to aggregate an electric charge on an empty metal globe on the highest point of a protected section, making high electric possibilities. It delivers high voltage coordinate current (DC) power at low current levels.

the first box is correct

The two dry sand samples are examined by changing one parameter. The independent variable, the one that is intentionally manipulated between the two samples, is the  Mass of each sample

the second box as well

Besides the amount of material, there is also another variable that is manipulated within each of the two mass groups and represented as three types. This independent variable is the type of material used

& the third box

In each case, the dependent variable, the one that you measure the response in, is the temperature over time