20. Given $a, b, c \in \mathbf{Z}_{+}, [a, b]=12, [b, c]$ $=15$. Then the minimum possible value of $[a, c]$ is ( ).
(A) 20
(B) 30
(C) 60
(D) 120
(E) 180
20. A.

From $[a, b]=12 \Rightarrow b \mid 12$.
From $[b, c]=15 \Rightarrow b \mid 15$.
To find the minimum value of $[a, c]$, i.e., $b$ should be as large as possible, thus, $b=3$.
At this point, $a=4, c=5, [a, c]=20$.