18. Correct. The answer is true

(a) If the area to the right of t₁ =0.05, then the area to its left is 0.95 and t₁ represents the 95^th percentile, which, according to the table of the t-distribution, is equal to 1.83.

(b) If the area to the left of -t₁ and to the right of t₁ =0.05, then the area to the right of t₁ is 0.025 by symmetry. Thus the area to the left of t₁ is 0.975 and t₁ represents 97.5^th percentile, or 2.26.

(c) If the area between -t₁ and t₁ is 0.99, then the area of the complement if 0.01 and the area to the right of t₁ is 0.005. Thus t_0.995 =3.25.

(d) If the area to the left of -t₁ is 0.01, then by symmetry the area to the right of t₁ is 0.01. Since t_0.99=2.82, the value of t for which the area to the left is 0.01 is -2.82.

(e) If the area to the left of t₁ is 0.90, then t_0.90=1.38.