An incident wave D=(2.0 cm) sin (4.5x-18t) (where x and t are in meters and seconds ) travels down a cord from a section with linear density 0.30 kg/m to one with density 0.10 kg/m. When the wave travels into the lighter density region, its wavelength becomes
a) 0.81m
b) 1.4m
c) 1.7m
d) 2.4m