Disease-carrying mosquitoes can spread diseases without affecting themselves. Nearly 700 million people get a mosquito-borne illness each year, which results in over one million deaths. Humans experience continuous pressures from disease-carrying mosquitoes in many parts of the world, so we have to find ways to fight against those insects because they keep getting scarier.
Even though DEET remains the most commonly used,and most powerful,mosquito repellent（防护剂）ever developed,scientists are actively pursuing effective products based entirely on plant oils. While DEET is an effective contact repellent, many people dislike the oily feel and smell on their skin,and sometimes some people are sensitive to it. Consumers are always interested in alternatives to DEET and other synthetic（合成的） repellents, so there are numerous natural repellents on the market.
In my lab at Iowa State University,my team and I have been conducting research on natural products as possible insect repellents for the past 20 years. In the past few years,three of my Ph. D. students have contributed greatly to our research by designing, synthesizing, purifying, characterizing and testing more than 300 closely related chemicals coming from the best of natural repellents like citronellol, menthol and thymol.
These students have successfully tested these repellents against three species of dangerous mosquitoes. The first group of the new repellents act through the air. These chemicals have a vapor action that provides protection. These are potentially most useful in backyards, parks, houses, barracks and horse barns. The other group of the new repellents are the classic ones that stop insects from standing on a treated surface,such as human skin, clothing or tents； some researchers have conducted testing on some humans to confirm the effectiveness and identify the very best ones.
The new repellents were designed and made from the natural materials in plant essential oils. They maintain many of the advantages of the natural repellents：They are fully biodegradable（可生物降解的），with no ecological concerns or environmental wastes,and generally considered safe like the thousands of types of plant essential oils used in the flavor and perfume industries. However,thorough testing will be conducted to determine if they are truly non-poisonous because there is still no enough evidence.



What does the author mainly want to show in Paragraph 1？ ______

A. The way mosquitoes spread disease.
B. The high death rate of mosquito-borne illness.
C. The difficulty of fighting disease-carrying mosquitoes.
D. The urgency of finding tools to fight against mosquitoes

What is a disadvantage of DEET？ ______

A. It causes discomfort to the users.
B. It greatly harms people's health.
C. It can't be applied universally.
D. It won't be effective for long.

What can we learn about the second group of the new repellents？ ______

A. They can kill mosquitoes indirectly.
B. They are mainly used in the open air.
C. They are more effective on human skin.
D. They can prevent mosquitoes from contacting users.

What's the author's attitude to the new repellents？ ______

A. Disapproving
B. Negative
C. Objective
D. Subjective