A half-adder is a combinational circuit that adds exactly how many input bits, producing a SUM and a CARRY output?

Introduction / Context:
Adders are the core of arithmetic logic units (ALUs). The simplest adder is the half-adder, which demonstrates how binary addition creates both a sum and a carry. Knowing its exact input count is essential before graduating to full-adders and multi-bit adders.

Given Data / Assumptions:

• Half-adder has no carry-in input.
• Outputs are SUM and CARRY.
• Inputs are equal-weight bits (no prior carry).

Concept / Approach:
A half-adder adds two single-bit inputs (A and B). The SUM output is A XOR B, and the CARRY output is A AND B. Because there is no carry-in, it cannot properly serve as an intermediate stage in a ripple-carry chain except at the least significant bit. For all higher bits where a carry-in exists, a full-adder is needed (adds three inputs: A, B, Cin).

Step-by-Step Solution:
Define half-adder inputs: A, B → exactly two inputs.Define outputs: SUM = A XOR B; CARRY = A AND B.Conclude the input count: 2 bits.Choose “2.”

Verification / Alternative check:
Truth table confirms: for inputs 00, 01, 10, 11, the SUM and CARRY match binary addition without a carry-in, validating the two-input definition.

Why Other Options Are Wrong:
Values like 8, 10, or 16 describe word sizes, not half-adder inputs; “None” is invalid because “2” is correct.

Common Pitfalls:
Assuming a half-adder can replace a full-adder at non-LSB positions; forgetting that CIN must be accommodated by a full-adder.

Final Answer:
2