Training secondary school students as anti-smoke ambassadors using the service-learning model: A cluster randomized controlled trial with hybrid type 1 effectiveness-implementation design study protocol - Katherine Ka Wai Lam; Ka Yan Ho; Doris Yin Ping Leung; Arkers Kwan Ching Wong; Cynthia Sau Ting Wu; Charlotte Qi Liu; Ting Mao; Yim Wah Mak (PLoS ONE)