If a rider is attached to a bill, it must be introduced before what event?
a.) first reading
b.) second reading
c.) debate
d.) vote

First reading

Explanation:

If a rider is attached to a bill, it must be introduced before the first reading. The first reading  of a bill refers to the moment when this is introduced to a legislature. This is called the "first reading" as there are various stages that a bill is subjected to as it goes through the process of becoming legislation.

a. the astrolabe could determine the correct latitudinal location of the ship