What was a key design change for hfc-134a a/c systems versus cfc-12 a/c systems?

Answer : According to Society of Automotive Engineering (SAE) standards the major key design change that was done for CF-12 A/C systems to HFC-134a A/C systems for the mobile A/C system service is that it provided with new service fittings which reduced the venting of refrigerants into the atmosphere and hence the mixing was also reduced with the ozone.

This new, unique and advanced quick modification has helped a lot in terms of maintaining low level of CFC emission in the environment.

n has four pairs of electrons in the valence shell, electrons repel one another ,n has 7 protons in its nucleus, and since n has 7 protons, it must fill the second shell, giving it 4 pairs of electrons. the electrons form 3 bonds and 1 lone pair of electrons. each pair of electrons repels the other pairs, so they are equally far apart.

so all of the above

the answer is definitely c.