Which sentence has a shift in verb tense?

the barrier will drop down every time that a train approaches the crossing.

if anyone knows the answer, she does.

as i came in the front door, eddie is going out the back door.

a row of ants had filed up the chair leg and was about to consume our lunch.

pls

The human genome project is devoted to mapping the general dna sequence of our species. this could lead to the development of new medicines, as well as the possibility of using gene therapy to treat certain diseases. however, there are some ethical issues surrounding the mapping of individual genomes. one concern is a) that your genes may change over time, making the project useless. b) that insurance companies could discriminate based on genetic make-up. c) that since this has never been done before, we should probably not do it now. d) that sequencing our individual genomes is so expensive, it is a counter-productive strategy.