Final answer:
Only the first and third sentences provided are grammatically correct, with 'They're rudeness is inexcusable!' containing a possessive error and the fourth sentence 'They're being rude again!' being correct.
Step-by-step explanation:
The sentences that are grammatically correct are 'There is no reason to be rude to one another.' and 'Is there any way they can get along?' The second sentence 'They're rudeness is inexcusable!' is incorrect because it should use the possessive 'Their' instead of 'They're', which is a contraction for 'they are'. The fourth sentence 'They're being rude again!' is correct in using 'They're' as it means 'they are'. In the review questions provided, comma placement and sentence structure play an important role in grammatical correctness.
Let's consider the example: Review question 10, 'Potassium and water is a dangerous combination!' The correct form of the verb is 'are' because the sentence talks about two items, potassium and water, so the plural verb 'are' should be used.