Để có hiện tượng sóng dừng trên một sợi dây, một đầu cố định, một đầu tự do, thì chiều dài của sợi dây thoả mãn ( $ k\in Z $ )
A. $ L=k\dfrac{\lambda }{2} $
B. $ L=\left( 2k+1 \right)\dfrac{\lambda }{2} $
C. $ L=\left( 2k+1 \right)\dfrac{\lambda }{4} $
D. $ L=k\dfrac{\lambda }{4} $

Điều kiện có sóng dừng trên dây chiều dài ℓ khi cả hai đầu dây cố định hay hai đầu tự do là
A.ℓ = kλ.
B.ℓ = (2k + 1) λ/4.
C.ℓ = (2k + 1) λ/2.
D.ℓ = kλ/2.

Một dây đàn hồi có chiều dài ℓ, hai đầu cố định. Sóng dừng trên dây có bước sóng dài nhất là
A.$λ_{max} = ℓ$.
B.$λ_{max} =2 ℓ$.
C.$λ_{max} = ℓ/2$.
D.$λ_{max} =4 ℓ$.

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
Drying flowers is the best way ________ them.
A.by preserving
B.preserved
C.to preserve
D.preserve

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
The policeman explained to us ________ get to the market.
A.how we could
B.how could
C.how could we
D.how

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each unfinished sentence.
His parents never allowed him _______.
A.smoke
B.to smoke
C.smoking
D.smokes

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
John proposed _______ Mary on a fine day at the crowded beach in their hometown.
A.getting married
B.that they married
C.to marry
D.to have married

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
The staff at the health center says that resting will help adults as well as children _________ from the common cold.
A.to recover
B.be recovered
C.has recovered
D.will recover

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.
The president has already instructed our branch manager in New York ______ annual sales figures by the end of December
A.reports
B.reporting
C.to report
D.reported

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to the following question.
My mother always tells me _______ .
A.not to leave my shoes on the carpet
B.not leave my shoes on the carpet
C.don’t leave my shoes on the carpet
D.not to leaves your shoes on the carpet