There are five constitutional isomers with the molecular formula C6H14. When treated with chlorine at 300°C, isomer A gives a mixture of two monochlorination products. Under the same conditions, isomer B gives a mixture of five monochlorination products, isomer C gives four monochlorination products, and isomer D gives a mixture of three monochlorination products. From this information, draw the structural formula of isomer D.