Problem: What is the missing first step of the following process:  -  These substances rise into the high atmosphere - They react with water and oxygen - This creates a more acidic rain - The rain with the high acid content falls.

A: Chemicals like sulfur dioxide and nitrogen are released into the air


Problem: What is the missing first step of the following process:  -  The lungs have a series of smaller and finer tubes and sacs - Air reaches the smallest and finest air sacs called alveoli - Oxygen travels from the alveoli into the blood - Oxygen-rich blood is carried to the heart - Oxygen-rich blood is pumped throughout the body by the heart - Cells take in the oxygen rich blood and release carbon dioxide rich blood - The carbon dioxide rich blood travels to the lungs - Carbon dioxide travels through the throat and is exhaled.

A: Air enters the nose or mouth and travels through the windpipe into the lungs


Problem: What is the missing first step of the following process:  -  Food begins to be processed by the digestive system - The pancreas releases enzymes to aid digestion - Digested food affects the amount of sugar in the blood - The pancreas releases one hormone to lower the blood sugar (glucose) level - The pancreas releases another hormone to raisethe blood sugar (glucose) level.

A: Food enters the body


Problem: What is the missing first step of the following process:  -  These pores are called stomata - Water is absorbed by the roots - It passes through vessels in the stem on its way to the leaves - Sunlight is absorbed by a green chemical in the leaves.

A:
Carbon dioxide from the air passes through small pores (holes) in the leaves