Which of the following immunization strategies is preferred for adults under the age of 65 years with immunocompromising conditions who require both PCV13 and PPSV23?

a. PCV13 first, followed by PPSV23 at least 8 weeks later.

b. PCV13 first, followed by PPSV23 at least 1 year later.

c. PPSV23 first, followed by PCV13 at least 8 weeks later.

d. PPSV23 first, followed by PCV13 at least 6 months later.