Tea and Coffee are perfect substitute of each other, given the price of Tea and Coffee being `100 and `200 per Kg. a consumer is prepared to buy 3 Kg. of each. If the price of tea remain same and the price of Coffee rises to `400 per kg. the demand for Tea goes to 6 Kg. and that of Coffee falls to 1Kg. The elasticity of substitution between Tea and Coffee is (a) 1 ; (b) 4 ; (c) 5 ; (d) 3