Read the following passage and mark the letter A, B, C, or D to indicate the best answer to each of the following questions from 18 to 27.

[I] Climate change is one of the most pressing challenges of our time, affecting ecosystems, biodiversity, and human communities across the globe. [II] Scientists warn that unless greenhouse gas emissions are significantly reduced, the Earth's climate may experience irreversible damage. [III] The consequences of climate change are widespread, impacting weather patterns, agricultural productivity, and health conditions worldwide. [IV]

One of the primary causes of climate change is the burning of fossil fuels, such as coal, oil, and gas, which releases large amounts of carbon dioxide (CO2) into the atmosphere. This gas traps heat, leading to a rise in global temperatures, a phenomenon known as global warming. Another major contributor is deforestation, as trees play a critical role in absorbing CO2. When forests are destroyed, that CO2 is released, adding to the atmosphere's greenhouse effect.

The impacts of climate change are already evident in various forms. Rising sea levels are causing coastal erosion and increasing the frequency of flooding, which endangers millions living in low-lying areas. Additionally, extreme weather events like hurricanes, droughts, and heatwaves are becoming more common and severe. These events not only damage infrastructure but also pose serious risks to human health and food security.

Addressing climate change requires a collaborative, global response. Many governments have joined international agreements, like the Paris Agreement, aiming to limit global temperature rise and reduce emissions. However, it is crucial for individuals and businesses to also play a part by reducing their carbon footprint through actions such as conserving energy, reducing waste, and supporting renewable energy sources.

(Adapted from "Climate and Human Impact")

The word "their" in paragraph 4 refers to ________. A. deforestation B. individuals and businesses C. fossil fuel combustion D. greenhouse gases