In a circle with centre $O$, $AB$ is a diameter and $CD$ is a chord such that $\angle CAB = 40^\circ$. What is the measure of $\angle ABD$ if chord $CD$ is parallel to diameter $AB$? Wait, if $CD \parallel AB$, then $\angle ABD = ?$

Options:

• A: 50°
• B: 40°
• C: 90°
• D: 60°

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