Put the correct Present Perfect, Simple past, or Simple Present Active or Passive verb form in the blanks. 1. The role of blood clots in hear attack (not yet determine) . But since a clot (block) blood flow to a part of the heart muscle, thereby killing it, researchers hope that removing a clot within the first hour will (resupply) the area with blood.
2. The outer surfaces of large meteorites (melt) rapidly. After passing air streams (sweep) this liquid rock away, a new cool layer of material (expose) . The molten liquid (create)
the blazing trail which (see) from the earth.

3. The earth’s internal forces of uplift are more active in the western U.S. than in the eastern part. Eruptions (occur) in this area for centuries. Mount St. Helens (erupt) in 1980. The Baldwin Hills area of the Los Angeles Plain (steadily push) upward at the rate of three feet every 100 years. There (be) also constant slipping along San Andreas fault.

4. Not only numbers but all characters (code) in binary when they (transmit) to the computer. The machine (give) a list of equivalent expressions to represent these symbols. The central processing unit (design) to recognize these binary numbers. The programmer, however, (must organize) and separate the information the computer needs and instructions it (must execute) in order to obtain accurate results for the program.

5. Paleontologists (discover) what (believe) to be the oldest form of life on earth. Life, in the form of bacterial cells, (exist) 3.5 billion years ago, only one billion years after the earth (form) . The fossilized bacteria (discover) by examining old sedimentary rocks (find) in Australia. About five different forms of bacteria (identify) that are almost identical to their modern-day cousins. The discovery (push) back the origin of life 1.2 billion years earlier than (previously think) .

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