6. As shown in Figure 1, each number in a box is determined by the product of the two numbers in the adjacent boxes of the upper layer (for example, $30=6 \times 5$). Then the number that should be filled in the unfilled box in the first row is ( ).
(A) $2 \quad$ (B) 3
$\begin{array}{lll}(\mathrm{C}) 4 & (\mathrm{D}) 5 & (\mathrm{E}) 6\end{array}$
6. C.

Let the unfilled number in the first row be $x$, and the unfilled number in the second row be $y$. According to the problem, we have
$$
\left\{\begin{array} { l } 
{ 6 0 0 = 3 0 y , } \\
{ y = 5 x }
\end{array} \Rightarrow \left\{\begin{array}{l}
x=4, \\
y=20 .
\end{array}\right.\right.
$$