Which is the electronic configuration for oxygen?
1s2 2s2 2p1
1s2 2s2 2p3
1s2 2s2 2p4
1s2 2s2 2p6

                     1s², 2s², 2p⁴

                      Electronic configuration is the distribution of electrons of an atom in atomic orbitals. As we know Oxygen is a non metal and is present in group 6 and period 2 with atomic number 8. The atomic number in fact specifies the number of protons. Hence, for a neutral oxygen atom there must be 8 electrons to balance the charges of protons.

                        These 8 electrons are distributed in two main energy levels i.e. n = 1 and 2 and sub energy levels i.e. s and p. According to certain rules like Aufbau Principle, Pauli's Exclusion Principle and Hund's Rule the electronic configuration for eight electrons is as,

                                           1s², 2s², 2px², 2p¹, 2p¹

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

explanation:

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.