题干

孟德尔在研究两对相对性状的杂交实验时,针对“重组现象”(F2中出现两种新类型)提出的假设是(     )

A:生物性状是基因控制的,控制不同性状的基因不相同

B:F1全部为显性性状,F2中有四种表现型,且比例为9:3:3:1

C:F1形成配子时,每对遗传因子彼此分离、不同对的遗传因子自由组合

D:若F1测交,将产生四种表现型不同的后代,且比例为1:1:1:1

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C

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    With busy lives, it can be hard to find time to volunteer. However, the benefits of volunteering are great to you, your family, and your community(社区).

    One of the better-known benefits of volunteering is the effect on the community. Volunteering allows you to connect with your community and make it a better place. Volunteering also strengthens your ties to the community and broadens your support network, exposing(使暴露) you to people with common interests, neighborhood resources, and fun and satisfactory activities.

    While some people are naturally outgoing, others are shy and have a hard time meeting new people. Volunteering gives you the opportunity to practice and develop your social skills, since you are meeting regularly with a group of people with common interests. Once you have the desire, it's easier to make more friends and contacts.

    Volunteering provides many benefits for both mental and physical health. You're doing good to others and the community, which makes you experience the pleasure of success. Your role as a volunteer can also give you a sense of pride and identity.

    Reducing the risk of depression(沮丧) is another important benefit of volunteering. A key risk factor for depression is social isolation. Volunteering keeps you in regular contact with others and helps you develop a solid support system, which in turn protects you against stress and depression when you're going through challenging times. Volunteering is good for your health whether you are young or old, but it's especially good in older adults.

    Volunteering offers you the chance to try out a new career without making a long-term commitment. It is also a great way to gain experience in a new field. Volunteering work can expose you to professional organizations or internship(实习)that could be of benefit to your career. That volunteer work is unpaid doesn't mean the skills you learn are basic. Many volunteering opportunities provide training. In addition, volunteering can also help you build upon skills you already have and use them to benefit the greater community.

    Volunteering is a fun and easy way to explore your interests and passions. Volunteer opportunities that match both your goals and your interest are most likely to be fun and satisfying for you. Doing volunteer work you find meaningful and interesting can be a relaxing escape from your day-to-day routines of work, school, and family commitments. Volunteering also provides you with renewed creativity, motivation, and vision that can be carried over into your personal and professional life.