optional
adj
ˈɒpʃənəl
adj
1
if
something
is
optional,
you
do
not
have
to
do
it
or
use
it,
but
you
can
choose
to
if
you
want
to
可选择的，非强制的;
BrE
three
optional
courses
三门选修课
The
other
excursions
are
optional.
其他的短途旅行是自选的。
optional
extra
Leather
seats
are
an
optional
extra.
皮坐椅是可选配件。