6. A 1.00-kg mass is attached to a horizontal spring. The spring is initially stretched by 0.100 m and the mass is released from rest there. After 0.500 s, the speed of the mass is zero. What is the maximum speed of the mass? (A) 0.326 m/s (B) 0.438 m/s (C) 0.510 m/s (D) 0.593 m/s (E) 0.628 m/s