The population of Ectenia is 100 people: 40 work full-time, 20 work half-time but would prefer to work full-time, 10 are looking for a job, 10 would like to work but are so discouraged that they have given up looking, 10 are not interested in working because they are full-time students, and 10 are retired. What is the number of unemployed?

a. 10

b. 20

c. 30

d. 40