I think THEY ARE a very good thing. (pural - the subject is electric vehicleS) THEY ARE friendly to THE environment. Also THEY use electrical charge as fuel, which is of low cost. (Alternative sentence: Also, they use electricity as a fuel and this costs less than traditional fuels, such as petrol). The only disadvantage is THEIR high price but in THE future when THEY become (no S) very common, their price will also become low.