Analysis of air bubbles trapped in fossils, such as in fossil amber, show that our air in earliest times contained about 35% oxygen. Today, the average oxygen content of our air is approximately 20%. In some larger, more polluted cities, oxygen content in the air has been measured at 12 - 15%.
There are 297 words which start with 'pen'. 3 letter words starting with 'pen': pen. 4 letter words starting with 'pen': pend. pens.
Words that Begin with the Letter C (with /k/ sound): 1  Activity Sheet: Say the name of each picture out loud and listen for the Cc (/k/) sound. 2  Activity Sheet: Modern Manuscript: Trace (print) the words that begin with the Letter C. 3  Activity Sheet: Standard Font: Trace (print) the words that begin with the Letter  C.