Which statement is CORRECT about cross validation in machine learning?

$$a$)$ In k$-$fold cross validation, each data point is contained in k folds.

$$b$)5-$fold cross validation always provides a better fit than $10-$fold cross validation.

$$c$)$ In k$-$fold cross validation, all k iterations use the same one fold as the testing set.

$$d$)5-$fold cross validation is less likely to lead to overfitting than $20-$fold cross validation.