What is the taxpayer's filing status for 2018 in each of the following situations?

a. Bill's wife died in 2017. Bill maintained a household for his two dependent children in 2018.

b. Bill is unmarried and lives in an apartment. He supported his aged parents, who live in a separate home. Bill provided over half of the cost of maintaining his parents' home and also provided over half of each of his parents' support.

c. Bill is unmarried and maintains a household for his 18-year-old daughter and her husband. Bill pays over half of the support of his daughter. The daughter files a joint return with her husband.