A 6-year old boy with 2 years history of asthma has well controlled under inhaled steroid for 3month. A moment ago，he felt breathlessness and begun wheezing after activity， which medication does he need most urgently?
A
Inhaled corticosteroid
B
Systemic Corticosteroids
C
Leukotriene modifiers
D
Inhaled Rapid-Acting Beta2-agonists
E
Oral Rapid-Acting Beta2-agonists