Which of the following statements correctly describes the relationship between the null hypothesis ($H_0$) and the alternative hypothesis ($H_a$) in hypothesis testing?
Select one:
O a. The alternative hypothesis always includes a symbol with an equal sign, such as $=$, $\le$, or $\ge$.
O b. The alternative hypothesis states that there is no difference or effect between variables.
O c. The null hypothesis always includes a symbol with an equal sign, such as $=$, $\le$, or $\ge$.
O d. The null hypothesis always includes a symbol of inequality such as $\ne$, $<$, or $>$.