In the space provided, write the letter of the sentence that best expresses the implied main idea of each of the following paragraphs. ____ 1 The earliest humans probably used the lengthening and shortening of shadows on the ground to measure the passage of time. 2 Later, the sundial was invented to tell time more precisely, but still by using the shadow principle. 3 The hourglass, a slightly more recent invention, measured time by allowing grains of sand to fall from one container to another. 4 In about the year 1300, a primitive clock was invented. 5 It had only an hour hand, but it became the most exact way yet to tell the time. 6 Since then, clocks have been so improved technically that today's clocks and watches can be depended upon to be quite precise.