$4\mathrm{.}8$ Suppose a $32-$bit instruction takes the following format: OPCODE DR SR$1$ SR$2$ UNUSED If there are $225$ opcodes and $120$ registers, a$.$ What is the minimum number of bits required to represent the OPCODE? b$.$ What is the minimum number of bits required to represent the destination register $($DR$)?$ c$.$ What is the maximum number of UNUSED bits in the instruction encoding