3 cm/sec3 \mathrm{~cm} / \mathrm{sec}3 cm/sec
60 cm/sec60 \mathrm{~cm} / \mathrm{sec}60 cm/sec
120.cm/sec120 . \mathrm{cm} / \mathrm{sec}120.cm/sec
180 cm/sec180 \mathrm{~cm} / \mathrm{sec}180 cm/sec