Which of these statements is true?

a.
some molecules may contain no atoms.

b.
some elements found in nature exist as molecules.

c.
some compounds include only one type of element.

d.
some atoms are made of two or more different elements

C is true.

There are some compounds that consist of two of the same elements forming homonuclear di-atomic molecules. However such are not considered as compounds per se. There are seven of these in the periodic table and they are all gases.  They are:

 Oxygen (O2), Colorless, odorless gas vital for life.

  Hydrogen (H2), The lightest known element.

 Fluorine (F2), Greenish yellow reactive toxic gas.

Chlorine (Cl2), Greenish yellow reactive toxic gas 

 Iodine (I2) Forms dense grey-black solid that produces purple vapors.

 Bromine (Br2). Forms dense red brown liquid with pungent odor.

They are referred to as di-atomic elements rather than di- atomic compounds.

the moving of the box is a result of centripetal force. think of that  as a car or a truck in this case creates a turn, the force of friction performing upon the turned wheels of the car runs centripetal force obligatory for circular motion for the things inside it.