参考答案：B

解析：Theophylline ethylenediamide is a xanthine derivative that acts directly on bronchial smooth muscle to relax and dilate the bronchi and relieve bronchial constriction and spasms. When the drug exerts its primary desired effect, dyspnea and shortness of breath decrease. Theophylline ethylenediamide does increase strength of myocardial contractility, but this is not the action for which it is used.