a 48-hour exposure time<br /><br />exposure time of 48 hours<br /><br />48 hours of exposure time<br /><br />an exposure time of 48 hours<br /><br />the exposure time totals 48 hours<br /><br />for 48 hours, the sample is exposed for 48 hours<br /><br />exposure should be 48 hours<br /><br />allocate 48 hours of exposure time<br /><br />