Weeds affect the crop plants by
A. Killing the crop plants before they grow
B. Secreting poisons which cause diseases in crop plants
C. Competing for various resources of crop plants causing low availability of nutrients
D. None of the above

Hint: Weed is a term referred to plants that grow in regions where they are of no use and are therefore not required. The problem of weeds is in existence ever since man started cultivating crop plants for food.

Complete Answer:
Weeds grow on farmland by feeding on nutrients present in the soil that is meant for the crop plant. Other than that weeds do not cause any other harm to crop plants.

Now, let us look at each option one by one.
- Weeds cannot kill crop plants before they grow. Therefore, this option is incorrect.
- Weeds are just normal plants that grow in places where they are not required. They do not secrete any poison to harm their surroundings. Therefore, this option is incorrect.
- Weeds indeed compete for various resources necessary for growth from the crop plants and deplete the nutrients available for the crop plants to grow. Therefore, this is the correct option.
- Since we do know that weeds compete for nutrients with crop plants, therefore, none of the above is the incorrect option.

Thus, the correct answer for the given question is (C) Competing for various resources of crop plants causing low availability of nutrients.

Note: There are 12 common types of weeds found in gardens and farms, these are-
- Wild violet
- Wild onion and garlic
- Weed tree saplings
- Dandelion
- Garlic Mustard
- Thistle
- Broad-Leaved Dock
- Bindweed
- Broadleaf Plantain
- Nutsedge
- Chickweed
- Yellow Wood Sorrel