A straight tube is sealed at one end, with a loudspeaker placed at the open end. The loudspeaker starts at a very low frequency and is then raised gradually. A sequence of loudness maxima is observed. The stationary wave that produces the first maximum has a node at the closed end and an antinode at the open end. The loudspeaker frequency is $f_1$ when the first maximum is observed. What is the loudspeaker frequency when the fourth maximum is observed?
- A$\frac{7f_1}{4}$
- B$2f_1$
- C$4f_1$
- D$7f_1$