[填空题]
A ball is thrown straight upnb0rrttw u+sze5p7 . ( with a speed of 30 m/s.jttrpjvx5k /(./ag. g (only fill in numerical value .k5t xv(tpg/. jrjag/in the blanks) (a.) How long does it take the ball to reach the maximum height? $s$ (b.) What is the maximum height reached by the ball? $m$ (c.) What is its speed after 4.2 s? $m/s$