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: Dental
Which of the following type of fissure is not self cleansable? 
Select and explain. 
A. I, B. IK, C. Inverted Y, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: MORPHOLOGY OF PIT & FISSURES
Nagano (1961) described following principal types of fissures, based on the alphabetical description of shape:

V – Type (34%)
U – Type (14%)
I – Type (19%)
IK – Type (26%)
Inverted Y – Type (7%)