Which sentence is in the passive voice?

a.
did you post the assignments on the board?

b.
a small fire in the hearth warmed the room.

c.
listen to the birds singing in the morning.

d.
an emergency meeting has been called for this afternoon.

invention of the washing machine

in the past, people used sticks and stones or scrub boards to clean their clothing. as it was sweaty, backbreaking work, we owe a world of to james king because he created the model for the modern washing machine in 1851. the most important issue was that this machine cleaned clothes while a person cranked the handle. doing this made people who used it develop strong muscles in their arms. later, in 1908, alva j. fisher created an electric washing machine named thor. this one was not the first automatic washing machine. however, it was one of the most popular models at the time.