Which one of the following elements must be available in sufficient quantity in the water of a pond in which tadpoles are growing?

Options

A. A. oxygen
B. B. iodine
C. C. nitrogen
D. D. chlorine

Explanation

To determine which element must be available in sufficient quantity in the water of a pond where tadpoles are growing, we can follow these steps:

### Step 1: Understand the Needs of Tadpoles
Tadpoles are the larval stage of frogs and require specific nutrients and elements in their environment for proper growth and development.

### Step 2: Identify Essential Elements
Tadpoles, like all aquatic organisms, need certain elements dissolved in water to support their metabolic processes. Key elements include oxygen, nitrogen, phosphorus, and other minerals.

### Step 3: Focus on Growth Requirements
During their growth, tadpoles need hormones such as thyroxin, which is crucial for their metamorphosis into adult frogs. Thyroxin is produced in the thyroid gland and is influenced by the availability of iodine in the environment.

### Step 4: Conclude with the Key Element
Among the various elements, iodine is particularly important for the production of thyroxin. Therefore, it must be available in sufficient quantities in the water of the pond for the healthy growth of tadpoles.

### Final Answer
Iodine must be available in sufficient quantity in the water of a pond in which tadpoles are growing.

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