[填空题]
A baseball is seen to pass upwardx7za3xj.bo y.p.qvl0r et;3i by someone on the 9th floor ewxv-)i*3bko+d02,e uev hj o(28 m above the street) with a vertica2 ohvvw,ee0io) udj+ 3kx*-b el speed of $20 \; m/s$. If the ball was thrown from the street, (a) what was its initial speed? $m/s$ (b) what maximum altitude does it reach? $m$ (c) how many seconds ago was the baseball thown from the street? $s$ (c) In how many seconds will it reach the street again? $s$
