A boat travels 1.2km upstream against the current at a constant speed of 3.0m/s, then immediately turns around and travels 1.8km downstream with the current at 6.0m/s. What is the boat’s average velocity for the entire trip? (Take downstream as the positive direction.)
(a) 4.29m/s downstream
(b) 0.86m/s upstream
(c) 0.86m/s downstream
(d) 1.5m/s downstream