This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Root canal anatomy and variation will be best known when one takes 
Select and explain. 
A. IOPA X-ray, B. IOPA with mesial angulation, C. IOPA mesial and distal angulation, D. Bitewing X-ray
C: IOPA mesial and distal angulation
Answer: C: IOPA mesial and distal angulation 
Explanation: Often a single radiograph of a multirooted tooth made at the normal vertical and horizontal projection does not display all the roots. In these cases, when it is necessary to separate the roots on multirooted teeth, a second projection may be made.
The horizontal angulation is altered,

20 degrees mesially for maxillary premolars.
20 degrees mesially or distally for maxillary molars.
20 degrees distally for an oblique projection of mandibular molar roots.

Ref: ORAL RADIOLOGY Principles and Interpretation,Stuart C. White,Michael J. Pharoah Edition 7 page no 129