Help?

Test Preparation SAT Preparation
1

In a circle, line AB and line CD are diameters. Which of the following must be true?

I. The length of arc ABD is equal to the length of arc BAC.
II. AB is perpendicular to CD.
III. ACBD is a square.

The answer was: A. I only

I understand why it can’t be II and III, but why is I the correct answer? I thought that you couldn’t have arc ABD. I know that you can have arc ADB, but ABD?

2

Arc ABD makes sense - it is the arc whose endpoints are A, D and contains B.