171.
If two solid hemispheres of same base radius are joined together along their bases, then curved surface area of this new solid is
Answer: B
Two hemispheres joined along their flat faces make a complete sphere, and the two flat faces vanish inside it.
\[ \text{curved surface} = 2 \times 2\pi r^{2} = 4\pi r^{2} \]
which is exactly the surface area of a sphere of radius \( r \). The \( 3\pi r^{2} \) option is the total surface of a single solid hemisphere, base included - but that base is no longer exposed once the two are joined.
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