The scientist knew nothing about the matter, because he was always _________ himself in his study.

A．burying

B．devoting

C．focusing

D．concentrating

将一白色固体与一溶液混合有大量气泡产生，则符合这一条件的下列组合是（　　）

A．二氧化锰与双氧水

B．碳酸钠粉末与稀盐酸

C．铁粉与稀硫酸

D．生石灰与稀盐酸等