Which of the following drugs has been found to be useful in acute severe asthma ?

A high concentration of oxygen should be given by face mask to achieve oxygen saturation of >90%.Magnesium sulfate given intravenously or by nebulizer has been found to be effective in acute severe asthma,when added to inhaled beta2 -agonists & is well tolerated.It can provide additional bronchodilation in patients whose presenting PEF(peak ejection fraction)is below 30% predicted. Reference:Harrison's medicine-18th edition,page no:2113;Davidson's medicine-22nd edition,page no:672.