An accident happens in the lab of Professor Utonium, and a radioactive element X is released in the form of a gas at around 4:00 am. Element X has a short half-life (30 min), and the lab would be considered safe when the concentration of X drops by a factor of 5. Considering the decomposition of element X is of first-order, what is the earliest time Professor Utonium can come back to do experiments in the lab? a) 4:09am b) 4:45am c) 7:36am d) 5:10 am e) 5:30am