Which of the following series of events is listed in proper sequence?
a. work begun on erie canal; work begun on national road; work begun on baltimore and ohio railroad; steamboat introduced on mississippi river
b. steamboat introduced on mississippi river| work begun on erie canal; work begun on baltimore and ohio railroad; work begun on national road
c. work begun on baltimore and ohio railroad; steamboat introduced on mississippi river; work begun on national road; work begun on erie canal
d. work begun on national road; steamboat introduced on mississippi river; work begun on erie canal; work begun on baltimore and ohio railroad?