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A full-adder can be implemented with half-adders and OR gates. A 4-bit parallel full adder without any initial carry requires
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- 8 half-adders,4-OR gates
- 8 half-adders, 3-OR gates
- 7 half-adders, 4-OR gates
- 7 half-adders, 3-OR gates
Correct Option: D
The circuit of a 4-bit full adder using half adders and OR gates is shown in the figure
From the figure, it is apparent that 4-bit full adder required seven half adders and 3-OR gates. 
