The sun was out and the temperature changing rapidly, it was really beginning to what?\nOptions:\n- drop to zero\n- increase\n- cool down\n- dropping quickly\n- heat up
Please answer the following question by reasoning step-by-step.
heat up
When the sun is out, the temperature begins to heat up.

Temperature becomes heat up when it changes rapidly after the sun is out.

.

Therefore, the answer is heat up