Read the excerpt from the dark game: true spy stories from invisible ink to cia moles.

"as montgomery and de grey slowly made their way through the message, they noticed more and more oddities. for example, 97556 appeared near the end of the message; the 9 family indicated important names that were not used very often in messages. we can imagine their shock when they realized that 97556 stood for zimmermann. that single name fired the men with excitement as they began working on the message from the beginning. in time, some of the coded "words" began to give up their secrets. they found most secret and for your excellency's personal information. the men pushed on, discovering mexico and japan in the text. what could that mean? and what was germany's interest in mexico? how did japan figure into the plan? the men could not think of a reason for the connection among germany, japan, and mexico. quickly thumbing the pages of the codebook, the men worked on at a fever pitch".

which word best describes the tone of the passage?

lighthearted
frustrated
suspenseful
frightened