A 26-year-old pregnant woman has been detected to have a diastolic murmur in mitral area. Echocardiography reveals mitral valve orifice to be 0.8 cm2. The cause of her murmur is
A
Mild mitral stenosis
B
Severe mitral stenosis
C
Functional murmur
D
Transmitted murmur from the congenital hea disease of the foetus