The grid in a high-vacuum triode is usually kept negatively charged with respect to the cathode so that
the electrons may be
a. accelerated toward the cathode.
b. attracted to the anode instead of the grid.
c. attracted to the cathode instead of the grid.
d. accelerated toward the anode.

The grid in a vacuum triode is usually kept negatively charged
with respect to the cathode so that the electrons may be
attracted to the anode instead of the grid. (B).

I'm pretty sure that nobody in your high school physics class
has ever seen a vacuum triode, or has ever used anything that
had vacuum triodes inside, or has the slightest idea of what a
vacuum triode is, or will ever need to know what a vacuum triode
is, or would realize any material benefit, now or in the future, from
that knowledge.  And I'd be very surprised if your time in high school
physics is being wasted to teach it to you.

Vacuum triodes were essential in virtually ALL electronic
equipment when I was in high school, but even then, it wasn't
taught until college.  I happened to learn about them on my own
while I was in high school, because that was when I got my
amateur radio ("ham") operator's license.

So how did you happen to run into this question ?

The rocks expand when there hot/warm  and contract when they are cold.