Describe how to find the least common multiple of three numbers

Find all the prime factors of the three numbers. pick up the common factors, ONCE, then pick up the non-common factors one by one, multiply the factors, the product is the least common factor.
example: the least common multiple of 6, 8, and 15
6=2*3
8=2*2*2
15=3*5
Note: do not write 8 into 4*2, because 4 is not a prime number. you have to break the number down to prime factors only. 
Notice that 6 and 8 have a common factor 2, so pick up the 2;
 6 and 15 have a common factor of 3, so pick up the 3.
those are the only two shared factors, so 2×3
now pick up whatever is not shared:
the two 2s for 8 and the 5 for 15 is not shared, add 2, 2, and 5 to the multiplication: 2×3×2×2×5=120
120 is the least common multiples of 6,8, and 15

this is basically how it is done. I believe you can explain better in your own words. 

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The answer to your question is b