[填空题]
A baseball is seen to pass upward n d(ilibwv.omy k7).7 b0a5pqby aam bbi8/ w/i9qyul )f9g:(t qsomeone on the 9th floor (28 m above the streetif:q bya8m (/9t)wbaq /guil9) with a vertical 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? $s$
