***Động năng bằng thế năng => Wđ = Wt => 2Wt = Wđ max
                                                         => 2*m*g*z = 1/2*m*v^2 max
                                                         => 2*m*10*z = 1/2*m*10^2
                                                         => 20*z = 50
                                                         => z = 2.5 (m)
***Thế năng bằng 4 lần động năng => Wt = 4*Wđ => Wt + 1/4*Wt = W
                                                                        => 5/4*Wt =Wđ max
                                                                        => 5/4*m*g*z = 1/2*m*v^2 max
                                                                        => 5/4*10*z = 1/2*10^2
                                                                        => z = 4 (m)
==================> Đáp án B