1.9. To determine in which test-tube the carbon dioxide concentration will decrease, we need to consider the processes of photosynthesis and respiration.

Here are the answers to the questions:

1.1.9. To determine in which test-tube the carbon dioxide concentration will decrease, we need to consider the processes of photosynthesis and respiration.

• Photosynthesis consumes carbon dioxide and produces oxygen, requiring light.
• Respiration consumes oxygen and produces carbon dioxide, occurring in both light and dark.

Let's analyze each test-tube:

• Test-tube A (Plant + Animal + No light): Both the plant and animal will respire, producing $\text{CO}_2$. The plant cannot photosynthesize due to lack of light. Therefore, $\text{CO}_2$ will increase.
• Test-tube B (Plant + No light): The plant will respire, producing $\text{CO}_2$. It cannot photosynthesize. Therefore, $\text{CO}_2$ will increase.
• Test-tube C (Plant + Light): The plant will photosynthesize (consuming $\text{CO}_2$) and respire (producing $\text{CO}_2$). Under light conditions, the rate of photosynthesis is typically greater than the rate of respiration, leading to a net consumption of $\text{CO}_2$. Therefore, $\text{CO}_2$ will decrease.
• Test-tube D (Animal + Light): The animal will respire, producing $\text{CO}_2$. There is no plant to consume $\text{CO}_2$. Therefore, $\text{CO}_2$ will increase.

Thus, the carbon dioxide concentration will decrease in test-tube C.

$$\boxed{\text{C}}$$

1.2.
1.2.1. A plant body with no true roots, stem and leaves: Thallus
1.2.2. A symbiotic relationship where both organisms benefit: Mutualism
1.2.3. Reproduction which uses non-reproductive parts of a plant to produce new plants: Vegetative propagation
1.2.4. The collective name for a filament and an anther: Stamen
1.2.5. Organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms: Humus
1.2.6. The type of animal that does not have a backbone: Invertebrate
1.2.7. Tissue that transports water in plants: Xylem
1.2.8. An underground stem in ferns: Rhizome
1.2.9. The incorporation of absorbed nutrients into the cells of the body: Assimilation
1.2.10. The process of producing ATP without oxygen: Anaerobic respiration