As Becca Thacher on "Life Goes On" (ABC, 1989-93) Martin established a strong and sympathetic identity on the acclaimed series. Over the course of four years, she developed from awkward child to assured young woman coping with family issues, school pressures and a romance with an HIV-positive boy. Martin brought intelligence and a natural ability to her performance and earned both critical praise and an Emmy nomination.She went on to head her own series, "Christy" (CBS, 1994-95), adapted from the Catherine Marshall novel, portraying a strong-willed and idealistic young woman who leaves her