6. $70 \%$ of the students in the class participate in the biology group, $75 \%$ participate in the chemistry group, $85 \%$ participate in the physics group, and $90 \%$ participate in the mathematics group. The percentage of students who participate in all four groups is at least ( ).
(A) $10 \%$
(B) $15 \%$
(C) $20 \%$
(D) $25 \%$
In fact,
$70 \%+75 \%=145 \%$, indicating that at least $45 \%$ of the students participated in both the biology and chemistry groups;
$45 \%+85 \%=130 \%$, indicating that at least $30 \%$ of the students participated in the biology, chemistry, and physics groups;
$30 \%+90 \%=120 \%$, indicating that at least 20\% of the students participated in all four groups.