A 1.0 m-long spring is laid horizontally with one of its ends fixed. When you pull on it with 50 N, it stretches to 1.2 m. (a) What is the spring's force constant (b) How much force is needed to compress it to 0.7 m?

• A. 250 N/m; 175 N
• B. 50 N/m; 15 N
• C. 250 N/m; 75 N
• D. 50 N/m; 35 N