Mean proportional of two algebraic expressions

1. A.
   $(a+b)^2 (a-b)^2$
2. B.
   $(a+b)^2(a-b)$
3. C.
   $(a+b) (a-b)^2$
4. D.
   $a^2-b^2$

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1. A.
   $\frac{12}{a^2}$
2. B.
   $12a^2$
3. C.
   $\frac{12}{a}$
4. D.
   $12a$

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