Nguyên tắc hoạt động của quang điện trở (LDR) dựa vào hiện tượng

A. quang dẫn

B. phát quang của các chất rắn

C. phát xạ nhiệt electron

D. quang điện ngoài

Đặt điện áp xoay chiều u = 100 2 cos 100 π t ( V ) vào hai đầu đoạn mạch mắc nối tiếp gồm điện trở thuần R = 50 Ω , cuộn cảm thuần và tụ điện có dung kháng Z C  thay đổi. Điều chỉnh  Z C lần lượt bằng 15 Ω , 50 Ω và 45 Ω  thì cường độ hiệu dụng qua mạch lần lượt bằng I 1 , I 2   và  I 3 . Nếu I 1 = I 2 = I  thì

A.  I 3 = 2 I

B.  I 3 < I

C.  I 3 = 2 A

D.  I 3 > I

Choose the word or phrase - A, B, C or D - that best completes the passage.

 The most dominant and reliable features of facial expressions provide a constant channel of communication. They can be shifty and evasive; convey hate, fear, and guilt; or (35) _____ confidence, love, and support.

Referred to as "mirrors of the soul" our eyes serve as the major decisive factor in (36) _____ the spoken words. The eyes of the man converse as (37) _____ as their tongues, with the advantage that the ocular dialect needs (38) _____ dictionary, but it is understood all over the world. When the eyes say one thing, and the tongue another, a practiced man relies on eyes. Except (39) _____ extremely shy individuals, most people look for social acceptance by studying the eyes of others. Eyes also can (40) _____ indicate a positive or a negative relationship. People tend to look longer and more often at the people whom they trust, respect and care about than at those whom they doubt or (41) _____. Normal eye dilation is not under control of the individual. Personally characteristics such as introversion and extroversion also influence eye behavior. Eye behavior seems (42) ____ particular importance and is generally used to indicate whether one is open to communication. This can be observed when a teacher asks the class a question: students who think they know the answer will generally (43) ____ at the teacher, (44) ____ students who do not know the answer will usually try to avoid eye contact

Điền vào ô 43

A. notice               

B. think                          

C. aim                            

D. look