The correct answer is:

693 cmÂ²

693 cmÂ²

**Explanation:**

Since the ribs are equally spaced, so the angle made by two consecutive ribs at the centre $=â€‹)Â°_{Â}=4=r=ÂcmÎ¸=$**=($\displaystyle \dfrac{45}{3600}$ Ã— $\displaystyle \dfrac{22}{7}$ â€‹Ã— 42 Ã— 42 )cmÂ² = 693 cmÂ²**