Which of the following numbers are written in scientific notation ?
a) 0.641×10^3
b) 2×10^1
c) 9.999 ×10^4
d) 4.38 ×5^10
a) 0.641×10^3
b) 2×10^1
c) 9.999 ×10^4
d) 4.38 ×5^10