Francine rolled a marble down a ramp and timed how long it took for the marble to reach the bottom of the ramp. After one trial, she doubled the height of the ramp and added a thick layer of wool cloth to the surface of the ramp. She conducted three more trials and after reviewing the data, Francine concluded that the height of the ramp had no effect on the speed of the marble rolling down the slope. Which of the following reasons best describes why her conclusion is unsupported?

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation:

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.