Which pair of elements will form a covalent bond?
hydrogen and chlorine
sodium and chlorine
copper and bromine
zinc and bromine

Answer is: hydrogen and chlorine.

Covalent bond is bond between nonmetals. Hydrogen (H) and chlorine (Cl) are nonmetals.

Hydrogen and chlorine will form hydrogen chlorine in chemical reaction:

H₂(g) + Cl₂(g) → 2HCl(g).

Other pairs will form ionic bonds, because one pair is metal (sodium, copper and zinc) and another pair is nonmetal (chlorine and bromine).

Ionic bond is the electrostatic attraction between oppositely charged ions (cations and anions).

answer

fusion: the energy source of stars. the energy released from the collapse of the gas into a protostar causes the center of the protostar to become extremely hot. when the core is hot enough, nuclear fusion commences. fusion is the process where two hydrogen atoms combine to form a helium atom, releasing energy.

hope it

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.