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<p><strong><a>A.</a></strong> <strong>ground level and springing line</strong> <strong><a>B.</a></strong> <strong>crown and springing line</strong> <strong><a>C.</a></strong> <strong>crown and ground level</strong> <strong><a>D.</a></strong> <strong>intrados and extrados.</strong></p> <p> </p>
The sagitta is the height of an arc. It is the perpendicular from the midpoint of the arc's chord to the arc itself. 
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He agreed with the colleagues' answers
I believe the best choice is answer "D" intrados and extrados.
The depth of an arch is the distance between intrados and extrados.
