A student is analyzing an unknown substance. The substance is solid at room temperature, but when heated only slightly, it melts. It boils when placed over a flame for more than a few seconds. The student dissolves the substance in water and finds the solution does not conduct electricity. Based on the experimental evidence, how should the student categorize the molecules that make up this substance? A:ionic compounds

B:transition metals

C:radioactive

D:covalent compounds