Baltes et al.'s research in gerontology is focused on plasticity. The research uses the term "plasticity" to describe people's capacity. Plasticity refers to the belief that capacity is not fixed but can be learned or improved with practice. Which core issue of human development does Baltes et al.'s research explore? a. stability–change issue b. universal versus context-specific controversy c. continuity–discontinuity controversy d. nature–nurture issue