For the following taxpayers, determine if they are required to file a tax return in 2018.
a. Ricko, single taxpayer, with gross income of $15,000.

b. Fantasia, head of household, with gross income of $17,500. 

c. Ken and Barbie, married taxpayers with no dependents, with gross income of $20,000.
d. Dorothy and Rudolf, married taxpayers, both age 68, with gross income of $25,500.
e. Janyce, single taxpayer, age 73, with gross income of $13,500.