[填空题]
A baseball is seen to pass upward b6v2 lprz:v.+ cf *txh lz/:nsvy someone on the 9th floor (28 m abo s alx.gt85)cjve the street) with a vertical st l)s.cg8 jax5peed 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? $s$
