Write as a decimal number:
a. It is estimated that the population of the world in 2020 will be 7.4×109 people.
b. The mass of a Ryukyu mouse is 1.12×10−2 kg.
c. The bacterium *bordelella pertussis* is about 5×10−7 m long.
d. The Eiffel tower in Paris weighs approximately 7.3×106 kg.