The heights (in cm) of steps in a newly built staircase form an arithmetic progression: the first step is 18 cm high, and each successive step is 0.5 cm shorter than the previous one. Steps are built until a step height of 15 cm is reached or crossed. Which statement about the resulting sequence of step heights is correct?
a) Finite AP with 6 terms
b) Not an AP because the common difference changes
c) Finite AP with 7 terms
d) Finite AP with 8 terms