Why do we only know a little bit about Pluto? a.
because it is too close to the Sun

b.
because aliens live in Pluto and they are not very welcoming

c.
because we don’t know where it is

d.
because it is too far away

Pluto is pretty far away from our planet, the Earth. It'd take a lot of years for the research and many years for the return.

So the answer is D.

For all other choices.

A is pretty wrong. As you should know that Pluto is VERY far from the Sun.

B is an assumption but not really that much logical.

For C, we know where Pluto is.

D is the most logical answer for the question as it is too far away for the research. After all It'd take more than approximately 10 years.

c is the answer because first she would blush to make the reader wonder why which they find out in the next line.

i think the first one, it makes the most sense.