unix - Converting strings in input file

I have a text file file1.txt on Unix. I'd like to produce another file file2.txt, in which I change all groups of lines that has this format (taken from a multiple-choice exam)

a. [first choice]
b. [second choice]
c. [third choice]

to

[first choice] [second choice] [third choice]

How could I do that?

EDIT: An example is

What is the value of three plus five?
a. six
b. seven
c. eight

This line is not so relevant.
blah blah

What is the capital of England?
a. London
b. Birmingham
c. New York

It should be converted to

What is the value of three plus five?
six seven eight

This line is not so relevant.
blah blah

What is the capital of England?
London Birmingham New York    

Answer

Assuming it's always 3 choices, a., b. and c., try this:

sed '/^[a-c]\. /{N;N;s/[a-c]\. / /g;s/[\r\n]//g;s/^ //}' file1.txt > file2.txt

This works by grabbing up three lines at a time using the N command, substituting a space for all the a., b. and c. occurrences, deleting all the line ends and, finally, deleting the last remaining extra space at the beginning of the line.