Answer: The sequence signal word that appears within the sentences is B. first.
Explanation: Signal words are used to connect the ideas presented in one or more sentences. Moreover, they can establish different relationships between ideas. For example, signal words can be used to indicate sequence, that is to say to show that some events take place before others. In the passage provided, the sequence signal word that appears within sentences is "first" since it shows that there is an order of events. In other words, "first" indicates that Chen decided to wash her hands before doing something else (eating).