What is the main difference between simultaneous and sequential games?

a$)$ In simultaneous games, players move one after the other, while in sequential games, players move simultaneously.

b$)$ In simultaneous games, players have complete information about each other's moves, while in sequential games, players do not.

c$)$ In simultaneous games, players move simultaneously without knowledge of each other's actions, while in sequential games, players observe previous moves before making their own.

d$)$ There is no difference between simultaneous and sequential games.