Three blocks a, b and c of masses 10 kg, 10 kg and 20 kg respectively are arranged as shown in figure. All the surfaces are frictionless and string is inextensible. A constant force F=20 N is applied on block a as shown. Pulley and string are light. Part of the string connecting both pulleys is vertical and part of the strings connecting pulleys with masses a and b are horizontal. Choose the correct option(s) for the given system.