1. The race does not take place every year because the ice must be at least 15 centimeters thick.
2. The race does not take place every year because the ice must be not less than 15 centimeters thick.
3. The race does not take place every year because the ice must not be less than 15 centimeters thick.
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Are they all the same? Which one is incorrect?
What other expressions can we use instead of 'take place' in the sentence?
All are correct, and all mean the same thing.
The race cannot be run ...
The race cannot occur ...