四、請說明全球化(globalization)的內涵，並如何以COVID-19疫情反思全球化的發展與限制？(25%)

(a) (10%) Suppose the sample size \( N \) follows a Poisson distribution with parameter \( \mu \), where \( \mu>0 \). Given \( N = n > 0 \), let \( S_n = X_1 + \cdots + X_n \) and \( S_0 = 0 \). Find the marginal distribution of \( S_n \). [Hint: \( P[S_n = 0|N = 0] = P[N = 0]. \)]