Step 1: Analyze the Assertion.
In a bi-party system, political power does indeed tend to alternate between two main political parties. This is the nature of the system, where the major political parties dominate the political scene, and one party's term in power is often followed by the other.
Step 2: Analyze the Reason.
The reason provided, stating that only two political parties exist in such countries, is incorrect. While a bi-party system typically involves two major parties, it does not necessarily mean that only two parties exist. Other minor parties may exist, but the political power alternates primarily between the two dominant parties.
Step 3: Conclusion.
Thus, while both statements are true, the Reason does not adequately explain the Assertion. Therefore, option (B) is correct.
Final Answer: (B) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion.