Which of these cannot be a source of centripetal force?
A. mass
B. friction
C. electric force
D. gravity

Explanation:

Gravity cannot be a source of centripetal force. Gravity is the force of attraction from between all masses in the universe; especially the attraction of the earth’s mass for bodies near its surface; “gravitation cannot be held responsible for people falling in love” with Albert Einstein.

It manner that is serious and solemn. A solemn and dignified feeling.

Therefore, the correct answer is d. gravity.

- "the acute form of diverticulitis can manifest itself with one or more severe attacks of infection and inflammation.

the inflammation of diverticulitis can eventually result in a bowel obstruction, which may cause constipation, thin stools, diarrhea, abdominal swelling or bloating, and abdominal pain."

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.