[填空题]
A baseball is seen to pass uppi) o.bqpci :07js3tea 6mtid:/ 6 mk8tgze7hward by someone on the 9th floor (28 m above the street) with a vem7 ma8tze6ht :/g dki6rtical 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? $s$
