Examine the following table of data, and then answer the question that follows.

Extract from the accounts of Sherston Logistics plc (Units $k)

Year	2007	2008	2009	2010	2011
Stock	100	120	140	160	160
Creditors	60	70	80	80	90
Cash	100	70	60	80	100
Overdrafts	60	80	100	110	120
Debtors	100	130	100	80	70
Short-term loans	40	40	30	40	60

For each year, calculate and comment on the firm's acid test ratio.