题目

Do you know why different animals or pests（害虫）have their special colours？Colours in them seem to be used mainly to protect themselves． Some birds like eating locusts（蝗虫），but birds cannot easily catch them．Why? It is because locusts change their colours together with the change of the colours of crops．Ⅵmen crops are green，locusts look green．But as the harvest time comes，locusts change to the same brown colour as crops have．Some other pests with different colours from plants are easily found and eaten by others．So they have to hide themselves for lives and a抛物线y=ax2+bx+c的图象向左平移2个单位，再向下平移1个单位，所得抛物线y=2x2-4x+3的解析式，则原抛物线解析式为（　　）A．y=2x2+4x+4B．y=2x2-12x+18C．y=2x2+4x+2D．y=2x2-12x+20