Cho phương trình $ x^2+y^2-2ax-2by+c=0\;(1) $ . Điều kiện để $ (1) $ là phương trình của đường tròn là
A. $ {{a}^{2}}+{{b}^{2}}-4c\ge 0 $ .
B. $ a^2+b^2-4c > 0 $ .
C. $ a^2+b^2-c > 0 $ .
D. $ {{a}^{2}}+{{b}^{2}}-c\ge 0 $ .

Đường tròn tâm $ I(1;\,\,4) $ và đi qua điểm $ B(2;\,\,6) $ có phương trình là
A.$ {{\left( x-1 \right)}^{2}}+{{\left( y-4 \right)}^{2}}=5 $
B.$ {{\left( x-1 \right)}^{2}}+{{\left( y-4 \right)}^{2}}=\sqrt{5} $
C.$ {{\left( x+1 \right)}^{2}}+{{\left( y+4 \right)}^{2}}=\sqrt{5} $ .
D.$ {{\left( x+1 \right)}^{2}}+{{\left( y+4 \right)}^{2}}=5 $ .

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
Tim had an interesting _______ year before going to university.
A.interval
B.between
C.gap
D.break

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
Please _______ clearly which courses you want to take.
A.instruct
B.ask
C.learn
D.indicate

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
Please leave this space _______ on the enrolment form.
A.blank
B.missing
C.absent
D.undone

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
A(n) _______ course allows students to manage their study schedule as long as they gain enough credits as required.
A.yearly
B.credit-based
C.monthly
D.distance

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
The tutorial system at Oxford and Cambridge is the _______ of many universities.
A.regret
B.envy
C.sorrow
D.jealousy

Chọn câu trả lời đúng trong các câu sau đây về thành phần của không khí ?
A.21% các khí khác, 78% khí nitơ, 1% khí oxi.
B.21% nitơ, 78% oxi, 1% các khí khác ( $ C{{O}_{2}},CO $ , khí hiếm,…).
C.21% khí oxi, 78% khí nitơ, 1% các khí khác ( $ C{{O}_{2}},CO $ , khí hiếm,…).
D.21% khí oxi, 78% các khí khác, 1% khí nitơ.

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
_______ learning allows students to work on their own at home and access materials and communicate with tutors and other students over a computer network.
A.Exchange
B.Distance
C.Postgraduate
D.Secondary

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to the following question.
In some countries, university students are selected _______ to their current level of academic attainment.
A.according
B.owing
C.due
D.relating