Which opening occurs in a pair?
(a) Obturator foramen
(b) Foramen magnum
(c) Foramen ovalis
(d) Fenestra rotundus

Options

A. Obturator foramen
B. Foramen magnum
C. Foramen ovalis
D. Fenestra rotundus.

Answer

To solve the question of which opening occurs in a pair, we will analyze each option provided:

1. **Obturator foramen (Option A)**:
- The obturator foramen is a large opening found in the pelvic girdle of humans.
- It is formed by the ischium and pubis bones.
- There are two obturator foramina, one on each side of the pelvis, meaning this opening occurs in a pair.

2. **Foramen magnum (Option B)**:
- The foramen magnum is the large opening at the base of the skull through which the spinal cord passes.
- There is only one foramen magnum in the human body, so this option does not occur in a pair.

3. **Foramen ovalis (Option C)**:
- The foramen ovalis (or oval foramen) is an opening in the heart, specifically in the interatrial septum.
- This opening is singular and does not occur in a pair, so this option is incorrect.

4. **Fenestra rotundus (Option D)**:
- The fenestra rotundus (or round window) is an opening in the inner ear.
- Similar to the foramen ovalis, this opening is also singular and does not occur in a pair.

After analyzing all options, we conclude that the correct answer is:

**Answer: (a) Obturator foramen**

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