What is one of the biggest drawback to using a conventional 3d printer for manufacturing parts? a. it can only print in metal materials such as aluminum first, which can then transferred to a plastic via blow molding b. printing fine geometry under 0.5mm is difficult to produce accurately unless very long build times are used which can take several hours c. the resulting part has much greater strength than if it was injection molded but it is much harder and therefore brittle d. only cylindircal shapes can be built in 3d, all others must be in 2d