For each of the following situations (none of the taxpayers claim dependents), indicate whether the taxpayer(s) is (are) required to file a tax return for 2018. Explain your answer. 

a. Helen is a single taxpayer with interest income in 2018 of $8,750. 

b. Joan is a single college student who is claimed as a dependent by her parents. She earned $1,550 from a part-time job and has $1,150 in interest income. 

c. Leslie, age 64, and Mark, age 66, are married and file a joint return. They received $17,800 in interest income from a savings account. 

d. Ray, age 60, and Jean, age 57, are married and file a joint tax return. Their only income is $14,700 in interest income. 

e. Harry, a 19-year-old single taxpayer, had net earnings from self-employment of $1,500.