What is sinus venosus?

Three chambered heart with sinus venosus, moist skin without scale and tympanum represents ear, are characteristics feature of :

Heart lacks sinus venosus in

A: In frog, sinus-venosus is present. R: In mammals and birds are remnant of sinus venosus has taken part in the formation of SA node.

Assertion: In frog, sinus venosus is present. Reason: In mammals and birds, the remnant of sinus venosus has taken part in the formation of SA node.

____Node may be regarded as a remnant of sinus venosus is mammalian heart.

What is correct about sinus venosus

The sinus venosus is located on

The sinus venosus and conus arteriosus in the heart of frog are present on which surface respectively