A.Directions : Read the following passages and choose the best answer for each blank.
    The past several centuries have witnessed pronounced increases in the height of populations all around the world. Houses built in New England 300 years ago seen quaint today because of the diminutive size of everything particularly the ….1….. of doorways. In 1920 the average student at Harvard University was one and a quarter inches taller and 10 pounds heavier than the average student had been a generation earlier.
     Scientists still have not pinpointed the exact ….2…. of this accelerated growth. Environmental changes such as improvements in public health, …..3…. and child care are questionably responsible to some degree, but there seem to be ….4.… factors at well For instance, ….5…. expansion takes relatively inbred country people and transport…..to cities, where they intermarry. Apparently the ….6…. of genetic stocks tends to produce bigger children. Therefore, industrialization and increases in the ….7.… of human beings seem to go hand in hand. A full ….8….. of the long-term trend to greater height, however, will undoubtedly disclose other ….9….. that are still unknown.