Given the fact "Earth 's tilt on its axis causes seasons to occur", what is the answer to the question or completion "It becomes cold enough for heavy clothing in December in the northern hemisphere because"

Options are:
[A]. our planet's axis is tilted
[B]. Mercury's axis is tilted
[C]. the gravitational pull of Pluto on the southern hemisphere
[D]. lyrics about winter by popular musicians always mention bundling up
[A].