The correct answer is:   [D]:   " 6√30 " .
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Explanation:
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Simplify:   " √20 ⋅ √3 · √18 " :
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Step 1)  Simplify the "first term" :
"√20  " = √4 √5 = 2√5  ; 
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Step 2):  Consider the "second term:  
                   "√3 " = √3  (already simplified); 
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Step 3)  Simplifly the "third term" :
 " √18  "  =  √9 * √2 =  " 3√2 " ;
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We have:  " 2√5 * √3 * 3√2 " ;
                  =   (2* 3) *  (√5 * √3 * √2) ;
                  =  6 * (√5 √3 √2)  ;
Note:  " (√5 * √3 * √2 )"  =   " √(5 * 3 * 2) "  =  " √ 30 " .
So:    " 6 * (√5 * √3 * √2)  =
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           →   " 6 √30 " .
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The answer is:  " 6√30 " ; 
        →  which is:  "Answer choice:  [D]:  " 6√30 " .
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Hope this answer helps!
Best wishes!
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