A 25.0-mL sample of an unknown HBr solution is titrated with 0.100 M NaOH. The equivalence point is reached upon the addition of 18.88 mL of the base. What is the concentration of the HBr solution?

a) 0.0755 M 

b) 0.0376 M

c) 0.100 M 

d) 0.00188 M